Wedgwood's  Pyroscope. 


Frontispiece^ 


The    Measurement    of 
High  Temperatures 


BY 

G.    K.    BURGESS 
*  i 

BUREAU  OF   STANDARDS 

AND 

H.    LE  CHATELIER 

MEMBRE  DE   L'lNSTITUT 

.  . 


THIRD  EDITION.    REWRITTEN  AND  ENLARGED 

FIRST    THOUSAND 


NEW  YORK 

JOHN    WILEY    &    SONS 

LONDON:  CHAPMAN  &  HALL,   LIMITED 

1912 


.11 


COPYRIGHT  1901,  1904,  1912, 

BY 
G.  K.    BURGESS 


Entered  at  Stationers'  Hall,  London 


Stanhope  ftress 

F.    H.  GILSON  COMPANY 
BOSTON,  U.S. A. 


PREFACE   I. 


THE  main  purpose  of  this  preface  is  to  recall  the  origin  of  the 
volume  Mr.  Burgess  and  I  present  to  the  reader.  For  a  long 
time,  all  precise,  scientific  investigations  at  high  temperatures 
were  made  impossible  by  the  absence  of  suitable  methods  for 
the  measurement  of  these  temperatures.  Wedgwood,  more  than 
a  century  ago,  had  already  insisted  on  the  capital  importance  of 
the  carrying  out  of  high-temperature  investigations,  and  devised 
for  this  purpose  his  pyrometer,  which  is  but  an  arbitrary-com- 
parison apparatus.  The  question  was  taken  up  later  by  many 
scientists,  but  with  little  success,  until  I  called  attention  defi- 
nitely to  the  precision  to  be  obtained  by  the  judicious  use  of 
thermoelectric  couples. 

Pouillet  about  1830,  and  afterwards  Edmond  Becquerel,  had 
made  some  measurements  with  the  gas  thermometer  provided 
with  a  platinum  bulb.  This  method,  however,  was  completely 
discredited  following  the  discovery,  by  Henry  Sainte-Claire 
Deville,  of  the  permeability  of  platinum  to  hydrogen;  and  it  is 
only  since  the  very  recent  employment  of  platinum-wound  elec- 
tric resistance  furnaces,  free  from  all  combustible  material,  that 
it  has  been  possible  to  obtain  accurate  measurements  with  the 
platinum-bulb  gas  thermometer;  but  its  complexity  and  the  dif- 
ficulty of  manipulation  limit  its  use  to  the  standardization  of 
other  measuring  apparatus. 

Henry  Becquerel,  later  his  son  Edmond,  and  also  Pouillet, 
advocated  the  employment  of  thermoelectric  couples;  but  their 
use  did  not  spread,  and  Regnault  emphatically  condemned 
them  after  finding  serious  irregularities  in  their  behavior. 
These  anomalies,  the  cause  of  which  he  did  not  then  recognize, 
were  due,  as  I  showed  later,  to  the  use  of  iron  as  one  of  the 


236339 


VI  PREFACE   I 

metals  of  the  couple,  and  to  imperfections  in  the  methods  of 
electrical  measurements. 

Violle,  following  Regnault  and  Sir  William  Siemens,  proposed 
the  calorimetric  method  with  platinum  as  the  heated  substance, 
instead  of  iron  used  by  Regnault  and  copper  by  Siemens,  in 
their  industrial  pyrometers.  This  complicated  method,  of  deli- 
cate and  slow  manipulation,  did  not  come  into  general  use. 

Mention  should  also  be  made  of  other  isolated  and  even  more 
restricted  attempts,  the  application  of  which  hardly  exceeded 
a  series  of  observations  by  a  single  experimenter.  Sir  William 
Siemens  proposed  the  electrical  resistance  pyrometer;  before 
this,  Edmond  Becquerel  had  suggested  a  radiation  pyrometer; 
finally,  several  observers  sought  to  apply  to  determinations  of 
the  sun's  temperature  certain  heat-radiation  methods. 

In  1885,  when  I  attacked  the  problem  of  the  measurement  of 
high  temperatures,  it  is  fair  to  say  there  existed  nothing  defi- 
nite available  on  this  important  question;  we  possessed  only 
qualitative  observations  for  temperatures  above  500°  C.  En- 
gaged at  that  time  in  industrial  studies  relative  to  the  manu- 
facture of  cement,  I  sought  a  method  which  above  all  would 
be  rapid  and  simple,  and  decided  on  the  use  of  thermoelectric 
couples,  intending  to  determine  the  order  of  magnitude  of  the 
sources  of  error  noticed  by  Regnault.  The  readings  of  even  a 
crude  galvanometer  might  be  very  useful  in  technical  work,  pro- 
vided the  limitations  of  its  accuracy  were  appreciated.  I  soon 
recognized  that  the  errors  attributed  to  this  method  could  easily 
be  eliminated  by  discarding  in  the  construction  of  the  couples 
certain  metals,  such  as  iron,  nickel,  and  palladium,  which  give 
rise  to  singular  anomalies;  and  indicated  a  simple  test  for  rec- 
ognizing the  suitable  metals.  One  takes  a  stretched  wire  of 
the  metal,  the  ends  of  which  are  connected  to  the  terminals  of  a 
sufficiently  sensitive  galvanometer,  indicating  at  least  T^^O  vo^> 
and  the  wire  is  then  heated  from  point  to  point  with  a  Bunsen 
flame,  which  is  carried  back  and  forth  beneath  the  wire,  when 
no  electric  currents  should  be  produced.  Now,  iron  and  pal- 
ladium, the  two  metals  advocated  by  Becquerel  and  Pouillet, 


PREFACE   I  vii 

give  rise  to  large  and  variable  parasite  currents  which  diminish 
the  accuracy  of  the  measurements.  Among  the  different  metals 
and  alloys  studied,  pure  platinum  and  the  alloy  of  platinum 
and  rhodium  which  are  still  used  to-day,  gave  the  most  satis- 
factory results. 

Finally,  I  called  attention  to  the  importance,  overlooked  by 
Regnault,  of  employing  only  galvanometers  of  high  resistance, 
to  avoid  the  influence  of  variations  in  resistance  of  the  wires  of 
the  couple  when  heated.  I  recommended  also  the  calibration 
of  the  couples,  not  against  the  air  thermometer  directly,  as  Bec- 
querel  had  tried  to  do,  but  in  terms  of  the  fixed  points  of  boiling 
or  fusion  of  certain  pure  substances,  in  such  a  way  that,  when 
these  temperatures  should  be  known  more  exactly,  as  is  the  case 
since  my  earlier  researches,  the  results  could  be  corrected  with 
certainty. 

Some  months  later,  at  the  request  of  Sir  Robert  Hadfield, 
director  of  the  Hecla  Steel  Works,  I  developed  an  optical  py- 
rometer, and  calibrated  it  by  comparison  with  the  thermoelectric 
couple.  By  means  of  these  two  instruments,  I  determined  a 
large  number  of  temperatures,  in  the  laboratory  and  in  the  in- 
dustries, and  rectified,  often  by  several  hundred  degrees,  the 
numbers  previously  admitted  in  terms  of  fantastic  estimations. 

From  this  date,  the  measurement  of  high  temperatures  came 
rapidly  into  general  use  in  the  laboratory  as  well  as  in  the  in- 
dustries. A  few  years  later,  in  a  course  of  lectures  delivered 
during  the  year  1898  at  the  College  de  France,  I  thought  it  use- 
ful to  give  a  summary  of  the  progress  accomplished.  These 
lectures,  gathered  into  book  form  with  the  aid  of  my  assistant, 
Mr.  Boudouard,  formed  the  first  edition  of  this  work.  Mr. 
Burgess,  who  had  followed  my  lectures,  took  the  trouble  to 
translate  it  into  English;  but,  while  there  was  little  demand  for 
the  French  edition,  the  English  translation  was  soon  exhausted. 
Mr.  Burgess  wrote  a  second  edition,  considerably  improved  and 
enlarged  by  him;  this  is  again  exhausted.  This  time  Mr.  Bur- 
gess has  rewritten  anew  the  whole  book,  so  that  it  is  no  longer  a 
translation  but  an  original  work  which  we  present  to  the  reader. 


VU1  PREFACE  I 

For  several  years  past,  my  studies  have  taken  me  into  other 
fields  of  investigation,  and  I  have  been  unable  to  follow  the  con- 
siderable progress  realized  in  the  measurement  of  temperatures. 
Mr.  Burgess,  on  the  contrary,  has  been  actively  interested  in 
these  new  researches  and  to  him  is  due  an  important  part  in  the 
more  recent  advances.  Consequently,  this  book  is  much  more 
his  work  than  mine,  which  enables  me  to  praise  it  as  it  deserves, 
and  state  that  this  publication  will  render  great  service  both  to 
investigators  and  engineers. 

H.  LE  CHATELIER. 

PARK,  February  15,  ign. 


PREFACE    II. 


SINCE  the  appearance  in  1900  of  Mesure  des  Temperatures 
Elevees  by  Messrs.  Le  Chatelier  and  Boudouard,  the  theory  and 
practice  of  pyrometry  have  grown  greatly,  and  methods  which 
at  that  time  were  in  a  tentative  stage  of  development  have 
been  improved  in  accuracy  and  convenience,  and  adapted  by 
means  of  new  instruments  both  to  technical  and  scientific  re- 
quirements. 

In  gas  pyrometry,  accurate  measurements  may  be  said  to 
have  been  initiated  at  the  Reichsanstalt  by  the  publication  in 
1900  of  a  series  of  metal  freezing  points,  by  Holborn  and  Day, 
constituting  what  is  still  known  as  the  Reichsanstalt  tempera- 
ture scale. 

Again,  it  is  only  since  1900  that  the  significance  of  the  applica- 
tion of  the  laws  of  radiation  to  pyrometry  has  been  appreciated. 
The  theoretical  work  of  Wien,  Planck,  and  others  closely  con- 
temporaneous with  the  experimental  verifications  of  Paschen, 
Lummer  and  Pringsheim,  and  many  others,  was  soon  followed 
by  the  optical  and  radiation  pyrometers  of  Wanner,  Fery, 
Morse,  and  Holborn  and  Kurlbaum,  and  by  many  applications 
to  technical  and  scientific  uses. 

In  thermoelectric  and  electric  resistance  pyrometry  there  has 
been  in  recent  years  an  unparalleled  improvement  in  the  design 
of  electric  measuring  instruments,  millivoltmeters,  potentiom- 
eters, galvanometers,  Wheatstone  bridges  and  the  like,  suitable 
for  use  in  temperature  measurements,  either  in  the  works  or 
in  the  laboratory.  There  have  also  been  executed  since  1900 
several  exact  experimental  investigations  in  pyrometry  with 
such  apparatus,  notably  at  the  several  national  laboratories  and 
at  the  Geophysical  Laboratory  of  Washington.  Furthermore, 


X  PREFACE   II 

the  subject  of  automatic  temperature  recording  has  received  a 
great  deal  of  attention,  resulting  in  many  new  instruments. 

In  so  far  as  practicable  in  the  following  pages,  we  have  dwelt 
less  upon  particular  types  of  instrument  than  on  the  principles 
underlying  them.  We  have,  however,  consulted  nearly  all  the 
manufacturers  of  pyrometers  as  to  their  practice,  and  have 
drawn  very  freely  on  the  material  they  have  been  so  kind  as  to 
put  at  our  disposal,  —  material  that  in  several  instances  is  other- 
wise unpublished,  and  for  which  we  express  our  obligation. 

We  have  kept  in  mind  three  classes  of  readers:  the  student, 
to  whom  the  historical  aspect  and  fundamental  principles  are  of 
prime  interest ;  the  engineer,  who  is  interested  mainly  in  adapting 
some  method  or  instrument  to  his  particular  technical  opera- 
tion; and  the  investigator,  who  has  an  intensive  interest  in 
accurate  methods  of  measurement  and  their  adaptability  to  his 
needs.  We  realize  that  one  book  cannot  meet  satisfactorily  all 
these  requirements.  If  the  wants  of  the  investigator  have  been 
somewhat  neglected,  he  has  ready  access  to  the  literature,  a 
summary  of  which  is  given  in  the  Bibliography. 

We  are  indebted  to  Dr.  C.  W.  Waidner  for  many  suggestions; 
to  Dr.  R.  B.  Sosman  for  reading  the  chapters  on  Gas  and  Ther- 
moelectric Pyrometry;  and  especially  to  Dr.  A.  L.  Day,  from 
whose  criticisms  of  the  manuscript  we  have  been  able  to  profit 
greatly. 

GEORGE   K.  BURGESS. 

WASHINGTON,  August  24,  ign. 


CONTENTS. 


INTRODUCTION. 

PAGE 

Thermometric  Scales 2 

Fixed  Points 5 

Pyrometers .- 9 

CHAPTER  I. 
STANDARD  SCALE  OF  TEMPERATURES. 

Laws  of  Mariotte  and  Gay-Lussac 13 

Gas  Thermometers 14 

Constant- volume  Thermometer.  .  . , 14 

Constant-pressure  Thermometer 15 

Thermometer  of  Variable  Pressure  and  Mass 15 

Volumetric  Thermometer 15 

Experiments  of  Regnault 16 

Results  Obtained  by  Chappuis 20 

Normal  Scale  of  Temperatures 21 

Thermodynamic  Scale 26 

Approximate  Expression 26 

Second  Approximation 28 

Gas-scale  Corrections 30 

The  Ice  Point 34 

CHAPTER  II. 

GAS  PYROMETER. 

Introduction 37 

Standard  Gas  Thermometer 38 

Formulae  and  Corrections 44 

Constant-volume  Thermometer 44 

Constant-pressure  Thermometer 50 

Volumetric  Thermometer 51 

Substance  of  the  Bulb 53 

Platinum  and  its  Alloys 54 

Iridium 56 

Iron 56 

Porcelain 57 

Glass 59 

Quartz 6a 

xi 


xii  CONTENTS 

PAGE 

Early  Experimenters 61 

Pouillet 61 

Ed.  Becquerel 62 

Violle 63 

Mallard  and  Le  Chatelier 65 

Barus 66 

Recent  Experimental  Investigations 67 

Holborn  and  Day 68 

Jaquerod  and  Perrot 70 

Calendar's  Constant-pressure  Thermometer 70 

Holborn  and  Valentiner 75 

Day,  Clement,  and  Sosman 75 

Comparison  of  Results 79 

Suggestions  for  Future  Experiments 80 

Methods 80 

Bulb 81 

Gas 82 

Manometer 82 

Industrial  Air  Pyrometers 82 

Indirect  Processes 83 

Methods  of  Crafts  and  Meier 83 

Methods  of  H.  Sainte-Claire-Deville 85 

Method  of  D.  Berthelot 85 

CHAPTER  III. 
CALORIMETRIC  PYROMETRY. 

Principle 89 

Choice  of  Metal OI 

Platinum OI 

Iron 02 

Nickel Q3 

Copper 03 

Calorimeters O4 

Industrial  Calorimeter Q5 

Siemens  Calorimeter 0,5 

Precision  of  Measurements Q7 

Conditions  of  Use oo 

CHAPTER  IV. 

THERMOELECTRIC  PYROMETER. 

Principle IOI 

Experiments  of  Becquerel,  Pouillet,  and  Regnault 101 

Experiments  of  Le  Chatelier  and  of  Barus I02 

Choice  of  the  Couple IO5 

Electromotive  Force IO,- 

Absence  of  Parasite  Currents IO5 

Chemical  Changes I0- 


CONTENTS  xiii 

PAGE 

Thermoelectric  Formulae 108 

Thermoelectric  Power 109 

Formulae m 

Platinum  and  its  Alloys in 

Variation  of  E.M.F.  with  Composition 116 

The  Base-metal  Couples 116 

Methods  of  Measurement  of  Temperature 116 

Galvanometric  Method 118 

Resistance  of  Couples  and  Galvanometer 118 

Pyrometer  Galvanometers 120 

Types  of  Suspended-coil  Galvanometers 124 

Pivot  Galvanometers 129 

Temperature  Coefficient  of  Galvanometers 131 

Galvanometer  Requirements  for  Industrial  Practice 133 

The  Galvanometer  Method  in  the  Laboratory 135 

Potentiometric  Methods 135 

Apparatus  Required 136 

Principle  of  the  Method 137 

Potentiometers  for  Use  with  Thermocouples 139 

The  Potentiometer  Indicator 140 

Precision  Requirements 141 

Types  of  Thermocouple  Potentiometer 143 

The  Thermocouple  Circuit 147 

Junction  of  the  Wires 148 

Annealing : 149 

Insulation  and  Protection 149 

Cold  Junction. '. 154 

The  Cold-junction  Correction !55 

Elimination  of  Cold-junction  Changes 156 

Constancy  of  Thermocouples 159 

Measurement  of  Inhomogeneity 162 

Reproducibility  of  Thermoelectric  Apparatus 164 

Base-metal  Thermocouples 165 

Nickel-copper 167 

Nickel-iron 168 

Complex  Alloy  Couples 170 

The  Noble  Metals:  GeibePs  Data 171 

Special  Couples 174 

Silver-constantan 174 

Silver-nickel 174 

Iridium-ruthenium 174 

Compound  Thermocouples 175 

Calibration  of  Thermocouples 178 

Precision  Calibration 178 

Crucible  Method 179 

Wire  Method 183 


xiv  CONTENTS 

PAGE: 

Use  of  Boiling  Points **7 

Technical  Calibrations l88 

Industrial  and  Scientific  Applications iQ1 

Conditions  of  Use JQ2 

CHAPTER  V. 
ELECTRICAL  RESISTANCE  PYROMETER. 

Introduction *94 

Work  of  Early  Investigators: 

Siemens *94 

Callendar  and  Griffiths iQ5 

Holborn  and  Wien - 196 

Law  of  Variation  of  Platinum  Resistance 19? 

Nomenclature • 201 

Construction  of  the  Platinum  Thermometer 202 

Choice  of  Size  of  Wire 206 

Precautions  in  Construction  and  Use 206 

Methods  of  Measurement 207 

Compensation  for  Pyrometer  Leads 208 

Three-lead  Thermometer 208 

Four-lead  Thermometer 211 

Wheatstone  Bridge  Method 211 

Precision  Bridges 212 

The  Potential-terminal  Thermometer 214 

The  Kelvin  Bridge 215 

Sensibility 216 

Direct-reading  Thermometers 218 

Harris  Resistance-thermometer  Indicator 218 

Logometer  and  Ratiometer 221 

Cambridge  Deflectional  Instrument 223 

Leeds  and  Northrup  Indicators 224 

Calibration 224 

Reduction  Tables 226 

Some  Results  Obtained 226 

Use  as  a  Standard 227 

Sources  of  Error  in  Accurate  Work 228 

Heating  by  the  Measuring  Current 228 

Lag  of  the  Platinum  Thermometer 229 

Insulation 229 

Compensation  for  Resistance  of  Leads 230 

Conduction  along  Leads 231 

Use  of  Impure  Platinum 231 

Changes  in  Constants 232 

Use  of  Metals  other  than  Platinum \  233 

Conditions  of  Use  of  Resistance  Pyrometer 234 

Industrial  Installations  and  Checking 234 


CONTENTS  XV 

CHAPTER  VI. 
THE  LAWS  OF  RADIATION. 

PAGE 

-General  Principles 238 

Temperature  and  Intensity  of  Radiation 238 

Emissive  Powers 239 

The  Black  Body 239 

Experimental  Realization 239 

Black-body  Temperature 242 

Kirchhoff's  Law 243 

Stefan's  Law 245 

Laws  of  Energy  Distribution 246 

Wien's  Laws 248 

Applications  to  Pyrometry .253 

CHAPTER  VII. 

RADIATION  PYROMETER. 

Principle '. 261 

Early  Investigators:  Temperature  of  the  Sun 262 

Pouillet's  Pyrheliometer 262 

Violle's  Actinometer 263 

Work  of  Rosetti 265 

Modern  Radiometric  Apparatus 267 

The  Thermopile 268 

The  Radiomicrometer 271 

The  Bolometer ; 273 

The  Radiometer , 275 

Standard  Pyrheliometers 276 

Pyrometric  Telescopes 277 

The  Fery  Pyrometer 277 

Fery  Mirror  Telescope .  .  281 

Fery  Spiral  Pyrometer 283 

Other  Radiation  Pyrometers 283 

Some  Experimental  Results 285 

Conditions  of  Use 286 

Calibration 288 

Computation 290 

CHAPTER   VIII. 

OPTICAL  PYROMETER. 

Principle 291 

Properties  of  Monochromatic  Radiation 291 

Methods  of  Temperature  Measurement 293 

Measurement  of  Total  Luminous  Intensity 293 


xvi  CONTENTS 

PAGE 

Measurement  of  the  Intensity  of  a  Simple  Radiation 295 

Optical  Pyrometer  of  Le  Chatelier 296 

Photometer 296 

Adjustment  of  Apparatus 299 

Measurements 299 

Details  of  an  Observation 300 

Emissive  Power 301 

Measurements  of  Intensity 302 

Calibration 302 

Evaluation  of  Temperatures 305 

Calibration  in  Terms  of  Wien's  Law 306 

Precision  and  Sources  of  Error 306 

Modifications  of  the  Le  Chatelier  Pyrometer 311 

Shore  Pyroscope 311 

Fery  Absorption  Pyrometer 311 

Wanner  Pyrometer 315 

Calibration 316 

Sources  of  Error. 320 

Range  and  Limitations 322 

Instrument  for  Low  Temperatures 324 

Holborn-Kurlbaum  and  Morse  Pyrometers 324 

Holborn-Kurlbaum  Form 325 

Morse  Thermogage 329 

Henning's  Spectral  Pyrometer 330 

Calibration  of  Optical  Pyrometers 331 

The  Wide-filament  Comparison  Lamp 332 

Use  of  Wedge-shaped  Cavities 333 

Monochromatic  Glasses 334 

Extension  of  Scale 336 

Some  Scientific  Applications 338 

Temperature  of  Flames 338 

Temperature  of  Glow-lamp  Filaments 340 

Temperature  within  Furnaces 342 

Melting  Points  of  Microscopic  Samples 343 

Conditions  of  Use ,44 

Some  Industrial  Uses 34^ 

Measurement  of  the  Relative  Intensity  of  Different  Radiations 346 

Use  of  the  Eye ; 346 

Use  of  Cobalt  Glass. 347 

Pyroscope  of  Mesure"  and  Nouel 34g 

Crova's  Pyrometer 3_0 

Use  of  Flicker  Photometer 3S2 

Stellar  Pyrometers 

Action  of  Light  on  Selenium 


CONTENTS  XVil 

CHAPTER  IX. 
VARIOUS  PYROMETRIC  METHODS. 

PAGE 

Wedgwood's  Contraction  Pyroscope 357 

Expansion  of  Solids 360 

The  Joly  Meldometer 361 

High-range  Mercury  Thermometers 362 

Fusing-point  Pyrometry 365 

Metallic  Salts 365 

Sentinel  Pyrometers 366 

Fusible  Cones 368 

Recent  Investigations  on  Seger  Cones 372 

Wiborgh's  Thermophones 376 

Dilution  Pyrometers 377 

Transpiration  Pyrometers 378 

Vapor-pressure  Pyrometers 380 

Other  Pyrometric  Methods 380 

CHAPTER  X. 
RECORDING  PYROMETERS. 

Forms  of  Temperature  Records 381 

Types  of  Cooling  Curves 383 

Methods  of  Recording 384 

Recording  Gas  Pyrometer 385 

Electrical  Resistance  Recording  Pyrometer 386 

Callendar's  Slide- Wire  Recorder 386 

Deflectional  Recorders 390 

Leeds  and  Northrup  Recorders 393 

Carpentier's  Electrothermal  Recorder ; 394 

Thermoelectric  Recording  Pyrometer 395 

Temperature-rate  Recorders 396 

Le  Chatelier's  Experiments 396 

Dejean's  Apparatus 399 

Temperature-time  Recorders 401 

Apparatus  of  Sir  Roberts-Austen 401 

Autographic  Recorders 408 

Semiautomatic  Recording 413 

Differential  Curves 414 

Use  of  a  Neutral  Body 414 

Saladin's  Apparatus • 418 

Registration  of  Rapid  Cooling 420 

Le  Chatelier's  Experiments 420 

Benedicks'  Experiments 421 

Recording  Radiation  Pyrometers 423 


xviii  CONTENTS 

PAGE 

Recording  Accessories 42^ 

Range  Control 426 

Multiple  Records  and  Circuits 428 

Furnace  Control  and  Thermostats 43° 

CHAPTER  XI. 
STANDARDIZATION  OF  PYROMETERS. 

Thermometric  Scales 432 

Fixed  Points 433 

Sulphur 433 

Zinc 437 

Gold 439 

Silver 44° 

Copper 44i 

Palladium 442 

Platinum 442 

Rhodium 444 

Iridium 444 

Other  Metals  melting  below  1100°  C : 445 

The  Iron  Group 445 

Metals  melting  above  2000°  C 446 

Typical  Freezing-point  Curves 446 

Boiling  Points:  Water,  Aniline,,  Naphthaline,  Benzophenone 447 

Metallic  Salts 450 

Alloys:  Eutectic  Points .• 450 

Reproducibility  of  Freezing  Points. 451 

Temperature  of  the  Arc  and  Sun. . .  ..^ 452 

Table  of  Fixed  Points 453 

Standardization  of  Pyrometejs 4156 

Standardizing  Laboratories - . ' 457 

Metals  and  Salts  of  Certified  Melting  Points 457 

Electrically  Heated  Furnaces 458 

Crucible  Furnaces 459 

Vacuum  and  Pressure  Furnaces 461 

Bibliography 465 

Appendix  :  Tables 489 

Index 499 


HIGH   TEMPERATURES 


INTRODUCTION. 

WEDGWOOD,  the  celebrated  potter  of  Staffordshire,  the  inven- 
tor of  fine  earthenware  and  of  fine  china,  was  the  first  to  occupy 
himself  with  the  exact  estimation  of  high  temperatures.  In  an 
article  published  in  1782,  in  order  to  emphasize  the  importance 
of  this  question,  he  considered  at  length  certain  matters  a  study 
of  which  would  be  well  worth  while  even  to-day. 

"The  greater  part  of  the  products  obtained  by  the  action  of 
fire  have  their  beauty  and  their  value  considerably  depreciated 
by  the  excess  or  lack  of  very  small  quantities  of  heat;  often  the 
artist  can  reap  no  benefit  from  his  own  experiments  on  account 
of  the  impossibility  to  duplicate  the  degree  of  heat  which  he  has 
obtained  before  his  eyes.  Still  less  can  he  profit  from  the  experi- 
ments of  others,  because  it  is  even  less  easy  to  communicate  the 
imperfect  idea  which  each  person  makes  for  himself  of  these 
degrees  of  temperature." 

Joining  example  to  precept,  Wedgwood  made  for  his  personal 
use  a  pyrometer  utilizing  the  contraction  of  clay.  This  instru- 
ment, for  nearly  a  century,  was  the  only  guide  in  researches  at 
high  temperatures.  Replaced  to-day  by  apparatus  of  a  more 
scientific  nature,  it  has  been  perhaps  too  readily  forgotten. 

Since  Wedgwood,  many  have  undertaken  the  measurement  of 
high  temperatures,  but  with  varying  success.  Too  indifferent 
to  practical  requirements,  the  early  experimenters  above  all 
regarded  the  problem  as  a  pretext  for  learned  dissertations. 
The  novelty  and  the  originality  of  methods  attracted  them  more 
than  the  precision  of  the  results  or  the  facility  of  the  measure- 
ments. Also,  up  to  the  past  few  years,  the  confusion  was  on  the 


"  HIGH  TEMPERATURES 

increase.  The  temperature  of  a  steel  kiln  varied  according  to 
the  different  observers  from  1500°  to  2000°;  that  of  the  sun  from 
1500°  to  1,000,000°.  More  recently  there  has  been  great  im- 
provement in  methods. 

First  of  all,  let  us  point  out  the  chief  difficulty  of  the  problem. 
Temperature  is  not  a  measurable  quantity  in  the  strict  sense  of 
the  term.  To  measure  a  length  or  a  mass,  is  to  count  how  many 
times  it  is  necessary  to  take  a  given  body  chosen  as  a  unit  (meter, 
gram)  in  order  to  obtain  a  complex  system  equivalent  either 
as  to  length  or  mass  of  the  body  in  question.  The  possibility  of 
such  a  measurement  presupposes  the  previous  existence  of  two 
physical  laws:  that  of  equivalence  and  that  of  addition.  Tem- 
perature obeys  well  the  first  of  these  laws;  two  bodies  in  tempera- 
ture equilibrium  with  a  third,  and  thus  equivalent  with  respect 
to  exchanges  of  heat  in  comparison  with  this  third  body,  will  also 
be  equivalent,  that  is  to  say,  equally  in  equilibrium  with  respect 
to  every  other  body  which  would  be  separately  in  equilibrium 
with  one  of  them.  This  law  allows  determination  of  temperature 
by  comparison  with  a  substance  arbitrarily  chosen  as  a  thermo- 
metric  body.  But  the  second  law  is  wanting;  one  cannot,  by  the 
juxtaposition  of  several  bodies  at  the  same  temperature,  realize 
a  system  equivalent,  from  the  point  of  view  of  exchanges  of  heat, 
to  a  body  of  different  temperature;  thus  temperature  is  not 
measured,  at  least  insomuch  as  one  considers  only  the  phenomena 
of  convection. 

In  order  to  determine  a  temperature  one  observes  any  phenom- 
enon whatever  varying  with  change  of  temperature.  Thus  for 
the  mercury  centigrade  thermometer  the  temperature  is  denned 
by  the  apparent  expansion  of  mercury  from  the  point  of  fusion  of 
ice  measured  by  means  of  a  unit  equal  to  T^  of  the  dilatation 
between  the  temperature  of  the  fusion  of  ice  and  that  of  the  ebulli- 
tion of  water  under  atmospheric  pressure. 

Thermometric  Scales.  —  For  such  a  determination  there  are 
four  quantities  to  be  chosen  arbitrarily:  the  phenomenon  meas- 
ured, the  thermometric  substance,  the  origin  of  graduation, 
and  the  unit  of  measurement;  while  in  a  measurement  properly 


INTRODUCTION 


so  called  there  is  but  one  quantity  to  be  arbitrarily  chosen,  —  the 
magnitude  selected  as  unity.  It  is  evident  that  the  number  of 
thermometric  scales  may  be  indefinitely  great;  too  often  experi- 
menters have  considered  it  a  matter  of  pride  for  each  to  have 
his  own. 

Here  are  some  examples  of  thermometric  scales  chosen  from 
among  many: 


Author. 

Phenomenon. 

Substance. 

Origin. 

Unit. 

Fahrenheit 

Dilatation 

Mercury 

(  Very  cold 
\     winter 

i/i8->  ice  to  B.  P. 

•p  ^ 

Dilatation 

Ice 

1/80  ice  to  B.  P. 

Celsius 

Dilatation 

Mercury 

Ice  

i/ioo  ice  to  B.  P. 

Wedgwood 

{Permanent             ) 
contraction         ) 

Clay 

Dehydrated 

i/24ooinit.  dimens. 

Pouillet 

Dilat.  at  const,  p. 

Air 

Ice 

(  Normal  ther.) 
(Therm  odynamic 
scale) 

Dilat.  at  const,  v. 
(  Reversible  heat    ) 
1     scale                    j 

Hydrogen 
Anything 

Ice 
Heat  =o 

i/iooice  to  B.  P. 

Siemens 

Electric  resistance 

Platinum 

Ice 

The  enormous  differences  above  mentioned  in  the  estimations 
of  high  temperatures  are  much  more  the  result  of  the  diversity 
of  the  scales  than  due  to  the  errors  of  the  measurements  them- 
selves. Thus  the  experiments  on  solar  radiation  which  have  led 
to  values  varying  from  1500°  to  1,000,000°  are  based  on  measure- 
ments which  do  not  differ  among  themselves  by  more  than  25 
per  cent. 

To  escape  from  this  confusion  it  was  first  necessary  to  agree 
upon  a  single  scale  of  temperatures;  that  of  the  gas  thermometer 
is  to-day  universally  adopted,  and  this  choice  may  be  considered 
as  permanent.  The  gases  possess,  more  than  any  other  state  of 
matter,  a  property  very  important  for  a  thermometric  substance, 
-  the  possibility  of  being  reproduced  at  any  time  and  in  any 
place  identical  with  themselves;  besides,  their  dilatation,  which 
defines  the  scale  of  temperatures,  is  sufficient  for  very  precise 
measurements;  finally,  this  scale  is  practically  identical  with  the 
thermodynamic  scale.  This  last  is  in  theory  more  important 
than  all  the  other  properties  because  it  is  independent  of  the 
nature  of  the  phenomena  and  of  the  substances  employed.  It 
gives,  too,  a  veritable  measure  and  not  a  simple  comparison;  its 


4  HIGH  TEMPERATURES 

only  inconvenience  is  for  the  moment  not  to  be  experimentally 
realizable,  at  least  rigorously,  but  this  will  probably  not  always 
be  the  case. 

The  adoption  of  the  scale  of  the  gas  thermometer  does  not  in 
any  way  imply  the  obligation  to  use  this  instrument  actually  in 
all  measurements.  Any  thermometer  may  be  taken,  provided 
that  in  the  first  place  its  particular  scale  has  been  standardized 
by  comparing  it  with  that  of  the  gas  thermometer.  According 
to  the  case,  there  will  be  advantage  in  employing  one  or  another 
method;  practically  also  one  almost  never  employs  the  gas  ther- 
mometer by  reason  of  the  difficulties  inherent  in  its  use,  which 
result  principally  from  its  great  dimensions  and  the  onerous 
manipulation  required. 

For  the  estimation  of  very  high  temperatures  the  gas  scale 
can  be  used  only  by  an  indirect  extrapolation  in  terms  of  some 
property  of  matter  whose  variation  has  been  studied  within  the 
range  of  the  gas  scale  attainable  experimentally  and  which  vari- 
ation is  assumed  to  obey  the  same  law  at  temperatures  beyond 
which  control  cannot  be  had  with  the  gas  thermometer. 

The  fact  that  certain  of  the  radiation  laws,  to  which  resort 
must  be  had  for  the  estimation  of  the  highest  temperatures,  have 
a  thermodynamic  basis  and  may  therefore  be  considered  an 
extension  of  the  thermodynamic  scale,  is  of  the  greatest  impor- 
tance in  the  extrapolation  for  temperatures  above  the  attainable 
limit  of  the  gas  scale. 

There  are  several  series  of  temperature  measurements  on  the 
gas  scale  in  good  agreement  to  1100°  C.,  and  two  series  reaching 
nearly  to  1600°  C.  which  differ  by  25°  at  this  temperature.  Be- 
yond 1600°  C.  the  most  infusible  substances  permanently  alter 
their  properties,  and  we  are  forced  to  measure  temperature  in 
terms  of  the  radiations  coming  from  heated  bodies  for  the  reason 
that  we  have  not  been  able  to  find  any  other  than  the  radiating 
properties  of  such  excessively  heated  bodies  whose  variations  can 
be  measured  without  destroying  or  permanently  altering  either 
the  substance  used  as  pyrometer  or  the  substance  examined. 
Perhaps  also  chemical  methods  may  be  employed  eventually. 


INTRODUCTION  5 

It  is  in  the  realm  of  the  laws  of  radiation  and  their  applications 
to  pyrometric  methods  that  some  of  the  most  recent  and  impor- 
tant advances  in  high-temperature  measurements  have  been 
made,  so  that,  with  certain  restrictions  which  will  be  treated  in 
the  chapter  on  the  laws  of  radiation,  it  is  possible  to  measure  on 
a  common  scale  the  temperatures  of  bodies  heated  to  the  highest 
attainable  limits. 

It  is  our  purpose,  in  this  introduction,  to  pass  in  review  rapidly 
the  different  pyrometric  methods  (that  is  to  say,  thermometers 
utilizable  at  high  temperatures)  whose  employment  may  be  ad- 
vantageous in  one  or  another  circumstance;  we  shall  then  de- 
scribe more  in  detail  each  of  them,  and  shall  discuss  the  conditions 
for  their  employment.  But  in  the  first  place  it  is  necessary  to 
define  within  what  limits  the  different  scales  may  be  compared 
to  that  of  the  normal  gas  thermometer;  it  is  the  insufficiency  of 
this  comparison  which  is  still  to-day  the  cause  of  the  most  im- 
portant errors  in  the  measurement  of  high  temperatures. 

Fixed  Points.  —  The  standardization  of  the  different  pyrom- 
eters is  the  most  frequently  made  by  means  of  the  fixed  points 
of  fusion  and  ebullition  which  have  been  determined  in  the  first 
place  by  means  of  the  gas  thermometer;  the  actual  precision  of 
the  measurements  of  high  temperatures  is  entirely  subordinate 
to  that  with  which  these  fixed  points  are  known;  this  precision 
was  for  a  long  time  most  unsatisfactory  because  these  fixed  points 
could  only  be  determined  in  an  indirect  manner  with  the  gas 
thermometer,  and  some  of  them  only  by  aid  of  processes  of  ex- 
trapolation, always  very  uncertain.  Recent  researches,  however, 
by  various  observers,  in  which  improved  methods  of  heating 
have  been  used,  as  well  as  greater  purity  of  materials  and  more 
carefully  constructed  and  calibrated  apparatus,  have  led  to  much 
more  concordant  results,  in  the  determination  of  fixed  points, 
even  by  most  varied  methods. 

Violle  was  the  first  to  make  a  series  of  experiments  of  consider- 
able temperature  range,  which  up  to  the  last  few  years  were  our 
most  reliable  data  on  the  question.  In  a  first  series  of  researches 
he  determined  the  specific  heat  of  platinum  by  direct  comparison 


6  HIGH  TEMPERATURES 

with  the  air  thermometer  between  the  temperatures  of  500°  and 
1200°.  He  made  use  indirectly  of  the  relation  thus  established 
between  specific  heat  and  temperature  to  determine  by  compari- 
son with  platinum  the  points  of  fusion  of  gold  and  silver;  then, 
by  extrapolation  of  this  same  relation,  the  points  of  fusion  of 
palladium  and  of  platinum. 

(   Ag*  Au  Pd  Pt 

Jl°n  ......................  \954°        1045°       1500°        1779° 

Finally,  in  a  second  series  of  experiments,  he  determined  by 
direct  comparison  with  the  air  thermometer  the  boiling  point  of 
zinc. 

Boiling  point...  . 


Barus,  when  physicist  of  the  United  States  Geological  Survey, 
determined  the  boiling  points  of  several  metals  by  means  of 
thermoelectric  couples  standardized  against  the  air  thermometer. 

(  Cd  Zn 

Boiling  point  ...........  \  7?2o  and  ?84o  9260  and  Q3i  o 

Mean  ...................  77#°  928.5° 

Callendar  and  Griffiths,  by  means  of  a  platinum  resistance 
thermometer  calibrated  up  to  500°  by  comparison  with  the  air 
thermometer,  have  determined  the  following  points  of  fusion 
and  ebullition: 

Sn  Bi  Cd  Pb  Zn 


FUSi°n \232°  270°  J2I' 

Boiling  point  under  (  Aniline     Naphthaline       Benzophenone      Mercury        Sulphur 

760  mm 1 184.1°       217.8°  305.8°          356.7°       444.5° 

These  last  figures  may  be  compared  with  Regnault's,  and  Crafts' 
previous  determinations  with  the  gas  thermometer: 

Naphthaline      Benzophenone      Mercury        Sulphur 
218°  306.1°  357°          445° 

Heycock  and  Neville,  employing  the  same  method,  but  with 
extrapolation  of  the  law  of  resistance  for  platinum  established 
at  that  time  only  up  to  450°,  determined  the  following  points  of 
fusion: 

232°     419°      633°       629.5°       654.5°        960.5°       1062°        1080.5° 

•  We  shall  use  figures  in  italics  for  all  fixed  points  determined  in  terms  of  the  gas  thermometer 
without  extrapolation. 


INTRODUCTION  7 

Also  using  the  platinum  thermometer  calibrated  at  o,  100,  and 
444.7°  C.  (the  sulphur  boiling  point),  Waidner  and  Burgess  more 
recently  at  the  Bureau  of  Standards  find  the  following: 

FREEZING  POINTS 

Sn  Cd  Pb  Zn  Sb  Al         Ag3-Cu2       Ag         Cu-CuzO         Cu 

231.9       321.0     327.4     419.4     630.7     658.0      779.2      960.9      1063.2      1083.0 

BOILING  POINTS 
Naphthaline 218.0°      Benzophenone 306-0° 

Jaquerod  and  Wassmer,  using  a  hydrogen  thermometer  of  66  c.c. 
bulb,  find: 

Boiling Naphthaline.  .  .  .217.7°      Benzophenone. .  .305.4° 

One  of  the  most  important  standardizing  temperatures  is  the 
boiling  point  of  sulphur  to  which  the  value  444-6°  C.  should  be 
assigned  from  the  work  of  some  half-dozen  investigators. 

At  the  Physikalisch-Technische  Reichsanstalt  the  question  of 
establishing  a  temperature  scale  has  received  deserved  attention. 
In  the  early  nineties  Holborn  and  Wien,  using  a  thermocouple 
calibrated  in  terms  of  a  porcelain-bulb  nitrogen  thermometer 
as  far  as  1400°,  found  the  fusing  points: 

Ag  Au  Pd  Pt 


Fusi°n }970°         1072°  1580°  1780' 

These  results  for  Ag  and  Au  were  subsequently  found  to  be 
high  by  Holborn  and  Day,  who,  after  trying  porcelain,  worked 
with  a  platinum-iridium-bulb  nitrogen  thermometer  and  ther- 
mocouple, employing  electric  heating,  two  improvements  that 
greatly  increased  the  accuracy.  In  fact,  the  return  to  metal 
bulbs  in  place  of  porcelain,  and  the  introduction  of  electric  heat- 
ing in  place  of  gas,  may  be  considered  the  inauguration  of  modern 
gas  pyrometry.  Holborn  and  Day  determined  the  following  on 
the  constant- volume  gas  scale: 

.  (      Cd  Zn  Sb  Al  Ag  Au  Cu 

'  1  321.7°       419°     630.5°       657-5°     96l-5°     1064°     1084° 

This  scale,  commonly  known  as  the  Reichsanstalt  scale,  was 
extended  to  1600°  by  Holborn  and  Valentiner,  who,  using  extrap- 


8  HIGH  TEMPERATURES 

olation  by  a  spectral  radiation  method,  obtained  the  further 
fixed  points: 

Fusion. Palladium  =  1575°       Platinum  =  1782° 

The  most  recent  determinations  (1911)  of  boiling  and  freezing 
points  are  due  to  Holborn  and  Henning: 

Naphthaline  =  217. o6,  Benzophenone  =  303.89,  Sulphur  =  444 .51 
Tin  =  231. 83,          Cadmium  =  320.9*,       Zinc  =  419.4$ 

Day,  Clement,  and  Sosman,  working  at  the  Geophysical  Lab- 
oratory of  the  Carnegie  Institution,  have  further  improved  the 
constant-volume  gas  thermometer  by  eliminating  the  pressure 
of  the  gas  on  the  bulb  and  substituting  a  Pt-Rh  for  a  Pt-Ir  bulb 
and  so  reducing  the  contamination  of  the  comparison  thermo- 
couples caused  by  evaporation  of  iridium.  Their  final  tempera- 
tures of  fusion  are  for  the  metals  they  studied: 

Cd     Zn     Sb    Al     Ag     Au      Cu     Ni    Co    Pd    Pt 
320.0  418.2  629.2  658.0  960.0  1062.4  1082.6  1452  1490  1549  1755 

Mr.  Daniel  Berthelot,  in  a  series  of  most  skillfully  executed  in- 
vestigations extending  over  several  years,  has  calibrated  thermo- 
couples by  comparison  with  a  special  form  of  gas  thermometer, 
making  use  of  the  variation  of  the  index  of  refraction  with  density. 
In  this  way  he  has  found  the  points: 

(    Ag  Au 

Fusion \o62°      1064° 

Se          Cd         Zn 
Ebullition 600°    778°    018° 

Besides  these  primary  measurements  there  are  some  very  im- 
portant secondary  determinations,  which  will  be  discussed  later. 

We  may  call  attention,  however,  at  this  point  to  some  of  the 
estimations  of  very  high  temperatures  obtained  by  extrapolating 
the  Reichsanstalt  scale  by  means  of  the  radiation  laws.  The 
palladium  and  platinum  melting  points  have  been  so  determined 
by  Nernst  and  Wartenberg: 

Palladium  =  1541°  Platinum  =  1745° 

Again,  Wartenberg,  using  a  vacuum  tungsten  resistance  furnace, 
finds  by  the  same  method: 

Ir  Rh  90  Pt-io  Rh  W 

2360°        1940°  1830°  2900° 


INTRODUCTION  9 

Finally,  Waidner  and  Burgess,  also  using  optical  methods,  find: 

Pd      Pt     Ta       W 
I5460   1753°   2910°    3050° 

Even  the  boiling  points  of  some  of  the  refractory  metals  have 
been  determined,  although  the  melting  points  are  to  be  preferred 
as  high-temperature  fixed  points.  We  may  cite  the  very  skill- 
fully executed  boiling-point  measurements  of  Greenwood  made 
with  an  optical  pyrometer : 

Al    Sb     Bi    Cr    Cu    Pe    Pb    Mg    Mn    Ag    Sn 
1800°  1440°  1420°  2200°  2310°  2450°  1525°  1120°  1900°  1955°  2270° 

From  all  the  results  at  hand  we  may  conclude  that  the  fixed 
points  possessing  the  greatest  reliability  for  the  indirect  stand- 
ardization of  the  various  thermometric  scales  and  thus  for  the 
calibration  of  pyrometers  are  the  following: 

Sn        Zn        Sb        Al        Ag         Au          Cu          Pd  Pt  Ir  W 


Fusion —  j  2^2~  4Iy.  03J.  05Q.  90I-  I00y  jotfj-  1550"  1755"  2300"  3000 

(      Naphthaline  Benzophenone          Sulphur 

Ebullltlon \         218.0°  306.0°  444.6° 

We  may  consider  the  high-temperature  scale  as  known  with 
an  accuracy  better  than: 

0.5° , between   200°  and  500°  C. 


2. 

3- 

15- 

25- 

So. 

100. 


500 
800 

1 100 

1600 

2OOO 
24OO 


800 
1 100 

1600 

2000 
2400 
3000 


A  more  detailed  discussion  of  the  determination  of  fixed  points 
and  their  reliability  and  ease  of  reproduction  will  be  found  in 
Chapter  XI  on  Standardization. 

Pyrometers.  —  There  have  been  a  great  number  of  pyrometric 
methods  proposed,  among  which  we  shall  dwell  only  upon  those 
which  have  had  considerable  use  or  promise  to  be  useful. 

Gas  pyrometer  (Pouillet,  Becquerel,  Sainte-Claire-Deville, 
Barus,  Chappuis,  Holborn,  Callendar,  Day).  —  Utilizes  the 
measurement  of  change  in  pressure  of  a  gaseous  mass  kept 
at  constant  volume.  Its  great  volume  and  its  fragility  render 


10  HIGH  TEMPERATURES 

it  unsuitable  for  ordinary  measurements;  it  serves  only  to  give 
the  definition  of  temperature  and  should  only  be  used  to  stand- 
ardize other  pyrometers. 

Calorimetric  Pyrometer  (Regnault,  Violle,  Le  Chatelier,  Sie- 
mens).—  Utilizes  the  total  heat  of  metals,  platinum  in  the 
laboratory  and  nickel  in  industrial  works.  Is  to  be  recom- 
mended for  intermittent  researches  in  industrial  establishments 
because  its  employment  demands  almost  no  apprenticeship  and 
because  the  cost  of  installation  is  not  great. 

Radiation  Pyrometer  (Rosetti,  Langley,  Boys,  Fery).  —  Utilizes 
the  total  heat  radiated  by  warm  bodies.  Its  indications  are 
influenced  by  the  variable  emissive  power  of  the  different  sub- 
stances. Convenient  for  the  evaluation  of  very  high  tempera- 
tures which  no  thermometric  substance  can  withstand  (electric 
arc,  sun,  very  hot  furnaces),  or  when  it  is  not  convenient 
to  approach  the  body  whose  temperature  is  wanted.  Can  be 
made  self-registering. 

Optical  Pyrometer  (Becquerel,  Le  Chatelier,  Wanner,  Hol- 
born-Kurlbaum,  Morse). —  Utilizes  either  the  photometric  meas- 
urement of  radiation  of  a  given  wave  length  of  a  definite  por- 
tion of  the  visible  spectrum,  or  the  disappearance  of  a  bright 
filament  against  an  incandescent  background.  Its  indications, 
as  in  the  preceding  case  but  to  a  much  less  degree,  are  influenced 
by  variations  in  emissive  power.  The  intervention  of  the  eye 
aids  greatly  the  observations,  but  diminishes  notably  their  pre- 
cision. This  method  is  mainly  employed  in  industrial  works  for 
the  determination  of  the  temperatures  of  bodies  difficult  of 
access  —  for  example,  of  bodies  in  movement  (the  casting  of  a 
metal,  the  hot  metal  passing  to  the  rolling  mill).  Can  be  used 
to  estimate  the  highest  temperatures  and  is  the  best  method  for 
use  above  1700°  C.  in  laboratory  and  works. 

Electric  Resistance  Pyrometer  (Siemens,  Callendar,  Waidner 
and  Burgess).  —  Utilizes  the  variations  of  electric  resistance  of 
metals  (platinum)  with  the  temperature.  This  method  permits 
of  very  precise  measurements  to  1000°  C.,  but  requires  the  em- 
ployment of  fragile  apparatus.  It  merits  the  preference  for  very 


INTRODUCTION  II 

precise  investigations  in  laboratories.  As  a  secondary  instru- 
ment for  the  reproduction  of  a  uniform  temperature  scale  through- 
out the  range  in  which  the  platinum  resistance  thermometer  can 
be  used,  to  1000°  except  in  very  heavy  wire,  it  is  unsurpassed  in 
precision  and  sensibility.  It  is  also  now  constructed  in  conven- 
ient form  for  industrial  use. 

Thermoelectric  Pyrometer  (Becquerel,  Barus,  Le  Chatelier).  - 
Utilizes  the  measure  of  electromotive  forces  developed  by  the 
difference  in  temperature  of  two  similar  thermoelectric  junctions 
opposed  one  to  the  other.  In  employing  for  this  measurement 
a  Deprez-d'Arsonval  galvanometer  with  movable  coil,  one  has 
an  apparatus  easy  to  handle  and  of  a  precision  amply  sufficient 
for  industrial  and  many  scientific  uses.  With  a  potentiometer, 
an  instrument  is  obtained  of  the  most  considerable  precision, 
available  for  use  to  1600°  C.,  or  even  to  1750°  with  proper  pre- 
cautions. This  pyrometer  was  used  for  a  good  many  years  in 
scientific  laboratories,  before  it  spread  into  general  industrial  use, 
where  it  also  renders  most  valuable  service. 

Contraction  Pyrometer  (Wedgwood) .  —  Utilizes  the  permanent 
contraction  that  clayey  materials  take  up  when  submitted  to 
temperatures  more  or  less  high.  It  is  employed  to-day  only 
in  a  few  pottery  works. 

Fusible  Cones  (Seger). —  Utilize  the  unequal  fusibility  of  earth- 
enware blocks  of  varied  composition.  Give  only  discontinu- 
ous indications.  Such  blocks  studied  by  Seger  are  spaced  so 
as  to  have  fusing  points  distant  about  20°.  In  general  use  in 
pottery  works  and  in  some  similar  industries. 

There  are  a  number  of  other  pyrometers  which  have  been 
found  suitable  in  special  cases  or  which  for  one  reason  or  another 
have  been  found  convenient  in  some  particular  line  of  work. 
Some  of  these  we  shall  mention,  among  them  being  the  mel- 
dometer  (Joly),  interesting  to  the  chemist  or  mineralogist  for 
determining  fusing  temperatures  of  minute  specimens;  the 
various  industrial  instruments  based  on  the  relative  expan- 
sion of  metals  or  of  a  metal  and  graphite  used  in  air  blasts 
and  metal  baths;  and,  finally,  pyrometers  based  on  the  flow  or 


12  HIGH  TEMPERATURES 

on  the  pressure  of  air  or  vapor  (Hobson,  Uhling-Steinbart,  Job, 
Fournier). 

Recording  Pyrometers  (Sir  Roberts- Austen,  Callendar,  Le  Cha- 
telier,  Siemens  and  Halske).  —  Finally,  we  shall  describe  in  some 
detail  the  application  of  registering  methods  to  pyrometry  both 
for  technical  and  laboratory  installations,  —  a  field  that  has  been 
cultivated  very  intensively  in  recent  years. 


CHAPTER  I. 
STANDARD  SCALE  OF  TEMPERATURES. 

WE  have  seen  that  temperature  is  not  a  measurable  quantity: 
it  is  merely  comparable  with  respect  to  a  scale  arbitrarily  chosen. 
The  normal  or  ideal  standard  scale  is  the  thermodynamic  scale ; 
but  as  it  is  impossible  to  realize  rigorously  this  scale,  it  is  neces- 
sary to  have  a  practical  one.  In  the  same  way  that,  besides  the 
theoretical  definition  of  the  meter,  there  is  a  practical  standard, 
a  certain  meter  kept  at  the  Bureau  International  des  Poids  et 
Mesures,  there  exists,  besides  the  ideal  scale  of  temperatures,  a 
practical  scale,  which  is  that  of  a  certain  gas  thermometer  which 
we  are  going  to  study.  We  shall  first  discuss  the  gas  laws  in  so 
far  as  is  necessary  for  our  purpose,  and  then  show  how  exactly 
these  laws  are  obeyed  by  the  actual  gases  that  may  be  used  in 
defining  the  temperature  scale  in  the  several  possible  ways. 

Laws  of  Mariotte  and  Gay-Lussac.  —  The  laws  of  Mariotte 
(or  Boyle)  and  of  Gay-Lussac  are  the  basis  for  the  use  of  the 
dilatation  of  gases  for  the  determination  of  temperatures.  These 
two  laws  may  be  written 

Pl'Ol  _  I  +<*/!  /    \ 

poV0          I+crfo' 

in  which  we  may  assume  for  the  present  that  the  temperatures 
are  being  measured  with  the  mercury  thermometer  from  o°  C. 
a  is  a  numerical  coefficient,  the  same  for  all  gases,  at  least  to  a 
first  approximation,  and  its  value  is  about 

a  =  0.00366  =  — - 
273 

when  it  is  agreed  that  the  interval  between  the  temperatures  of 
melting  ice  and  boiling  water  is  100°. 

But  instead  of  considering  the  formula  (i)  as  the  expression  of 

13 


14  HIGH  TEMPERATURES 

an  experimental  law  joining  the  product  pv  to  the  temperature 
denned  by  the  mercury  thermometer,  we  may  require  of  experi- 
ment merely  the  law  of  Mariotte  and  write  a  priori  the  formula 
in  question,  giving  a  new  definition  of  temperature  approximating 
that  of  the  mercury  thermometer.  This  new  scale  has  the  ad- 
vantage that  it  adapts  itself  to  the  study  of  very  much  higher 
temperatures.  The  use  of  this  process  suggested  by  Pouillet 
was  carefully  studied  by  Regnault  and  has  since  become  the 
most  common  method  of  defining  temperatures  practically. 

The  expression  for  the  laws  of  Mariotte  and  Gay-Lussac  can 
be  put  in  the  form 

.     .....     (2) 


by  calling  n  the  number  of  units  of  quantity  (this  unit  may  be 
either  the  molecular  weight  or  the  gram);  R  the  value  of  the 
expression 


for  unit  quantity  of  matter  taken  at  the  temperature  of  melting 
ice  and  under  atmospheric  pressure. 

Gas  Thermometers.  —  The  equivalent  expressions  (i)  and  (2), 
which  arbitrarily  by  convention  give  the  definition  of  temper- 
ature in  terms  of  the  elastic  properties  of  a  gas,  may  be  utilized, 
from  the  experimental  point  of  view,  in  various  ways  for  the 
realization  of  the  standard  thermometer. 

i.  Constant-volume  Thermometer.  —  In  the  thermometer  desig- 
nated by  this  name,  the  volume  and  the  mass  are  kept  invariable. 

The  expression  (2)  then  gives  between  the  two  temperatures 
t  and  to  the  relation 


A,     « 
from  which 


(3) 


STANDARD   SCALE   OF  TEMPERATURES  15 

2.  Constant-pressure  Thermometer.  —  In  this  case  the  pressure 
and  the  volume  of  the  heated  mass  remain  constant,  but  the 
mass  is  variable;  a  part  of  the  gas  leaves  the  reservoir.  The 
expression  (2)  then  gives 


from  which 


It  would  be  much  more  logical,  instead  of  the  classic  expressions 
constant-  volume  thermometer  or  constant-pressure  thermometer, 
to  say  thermometer  of  variable  pressure,  thermometer  of  variable  mass, 
which  describe  much  more  exactly  the  manner  of  their  action. 

3.  Thermometer  of  Variable  Pressure  and  Mass.  —  The  action 
of  this  apparatus  combines  those  of  the  two  preceding  types. 
A  part  of  the  gas  leaves  the  reservoir,  and  the  pressure  is  not 
kept  constant.  The  expression  (2)  gives 

i 

p   _  n      a 


_ 

po      n0     i        ' 
--Ho 


from  which 


4.  Volumetric  Thermometer.  —  There  exists  a  fourth  method 
of  the  use  of  the  gas  thermometer  which  was  suggested  by  Ed. 
Becquerel,  and  presents,  as  we  shall  see  later,  a  particular  interest 
for  the  evaluation  of  high  temperatures.  We  keep  the  name  for 
it  given  by  its  inventor.  The  determination  of  the  temperature 
is  obtained  by  two  measurements  made  at  the  same  temperature, 
and  not  as  in  the  preceding  methods  by  two  measurements  made 
at  two  different  temperatures  one  of  which  is  supposed  known. 


T6  HIGH  TEMPERATURES 

The  mass  contained  in  the  reservoir  is  varied,  and  the  ensuing 
change  of  pressure  is  observed.    The  expression  (2)  gives 


(,-,')  R 


from  which 


This  necessitates  a  preliminary  determination  of  the  constant  R. 
In  the  particular  case    in  which  p'  =  o,  which  supposes  that 
a  complete  vacuum  is  obtained,  the  preceding  relation  becomes 
simpler  and  is 

I  —  i  +  <.|  .......     (7) 

a      n      R 

The  definitions  of  temperature  given  by  these  different  ther- 
mometers would  be  equivalent  among  themselves  and  with 
that  of  the  mercury  thermometer  if  the  laws  of  Mariotte  and 
Gay-Lussac  were  rigorously  exact,  as  used  to  be  held,  and  if  the 
expansion  of  mercury  in  glass  were  linear.  The  only  advantage 
of  the  gas  thermometer  then  would  be  to  extend  to  high  tempera- 
tures the  scale  of  the  mercury  thermometer.  In  this  way  it  was 
employed  by  Pouillet,  Becquerel,  and  Samte-Claire-Deville. 

Experiments  of  Regnault.  —  The  very  precise  experiments  of 
Regnault  caused  a  modification  in  the  then  admitted  ideas  con- 
cerning the  mercury  thermometer  as  well  as  the  gas  thermometer, 
and  led  to  the  definite  adoption  of  the  gas  thermometer  as 
standard. 

In  the  first  place  these  experiments  established  that  different 
mercury  thermometers  are  not  comparable  among  themselves  on 
account  of  the  unequal  dilatation  of  the  differing  glass  employed 


STANDARD   SCALE  OF  TEMPERATURES 


in  their  construction.  Thus  they  cannot  give  an  invariable  scale 
for  the  determination  of  temperature.  In  comparing  them  from 
o°  to  100°  they  do  not  present  between  these  extreme  tempera- 
tures very  great  differences,  0.30°  as  a  maximum,  but  at  tempera- 
tures above  100°  these  differences  may  become  considerable  and 
reach  10°  to  20°  or  more.  (See  also  Chap.  IX.) 


Constant-vol. 

Mercury  thermometer  in 

0o=76o. 

Crystal. 

White  glass. 

Green  glass. 

Bohemian 
glass. 

100° 

+  0.00° 

+0.00° 

+0.00° 

+0.00° 

150 

+  0.40 

—  O.20 

+0.30 

+0.15 

2OO 

+    I.2S 

—  0.30 

+0.80 

+0.50 

250 

+  3-oo 

+0.05 

+  I-85 

+  1-44 

300 

+  5-72 

+1.08 

+  3-50 

350 

+  10.50 

+4.00 

The  numbers  figuring  in  this  table  indicate  the  quantities  by 
which  it  is  necessary  to  increase  or  diminish  the  temperatures 
given  by  the  air  thermometer  in  order  to  have  them  correspond 
with  those  which  were  observed  with  the  different  mercury 
thermometers. 

It  was  thus  impossible  to  define  the  practical  scale  of  tempera- 
tures in  terms  of  the  mercury  thermometer.  The  use  of  the  gas 
thermometer  became  necessary.  But  Regnault  recognized  that 
it  was  not  possible  to  take  a  single  coefficient  of  dilatation  a 
independent  of  the  nature  of  the  gas,  of  its  pressure,  and  of  the 
mode  of  dilatation  utilized.  The  coefficient  of  expansion  at  con- 
stant volume  ()8)  and  the  coefficient  of  expansion  at  constant 
pressure  (a)  are  not  identical.  This  follows  from  the  fact  that 
the  law  of  Mariotte  is  not  vigorously  exact;  we  have  in  reality 

pv  =  p0v0  +  c, 

e  being  a  very  small  quantity,  but  not  zero. 

The  experiments  of  Regnault  permitted  him  not  only  to  detect 
but  to  measure  this  variation  of  the  coefficient  of  expansion. 
Here  are,  for  example,  the  results  which  he  found  for  air  between 
o°  and  100°. 


1  8 


HIGH  TEMPERATURES 


Volume  constant. 


Pressure  constant. 


Pressure 

ft 

i 
ft 

Pressure 

a 

I 
a 

266 

0.003656 

273.6 

76o 

0.003671 

272.4 

76o 

3655 

272.8 

2525 

3694 

270.7 

1692 

3689 

271 

2620 

3696 

270.4 

36SS 

3709 

269.5 

For  air  at  4.5°  Rankine  obtained,  from  the  experiments  of 
Regnault,  the  formula 


pv  =  p0Vo  +  0.008163 


CO 


pv 


•w  being  the  atmospheric  pressure. 

These  coefficients  vary  also  from  one  gas  to  another,  as  is 
shown  by  the  following  table,  taken  also  from  Regnault's  experi- 
ments: 

MEAN   COEFFICIENT  BETWEEN  O°  AND  IOO°. 

Volume  constant.  Pressure  constant. 

Pressure.  R  i  Pressure  i 

mm.  tt  mm.  0 

Air, 

760   0.003665   272.8           760  0.003671  272.4 

3655      3709   269-5          2620  3696  270.4 

Hydrogen. 

760               3667        272.7                            760  36613  273.1 

2545  36616  273.2 

Carbon  Monoxide. 
760  3667        272.7  760  3669        272.5 

Nitrogen. 
760  3668        272.6 

Carbonic  acid. 

760  3688        271.2  760  3710         296.5 

3589  3860        259  2520  3845         259.5 

Sulphurous  acid. 

760  3845        259.5  760  3902         253.0 

980  3980        251.3 

These  experiments  show  that  the  easily  liquefiable  gases  have 
coefficients  quite  different  from  those  of  the  permanent  gases. 

For  the  permanent  gases  the  coefficients  for  constant  volume 
differ  much  less  among  themselves  than  those  for  constant 


STANDARD   SCALE  OF  TEMPERATURES 


pressure;  for  the  former  the  extreme  deviation  does  not  exceed 
ToVo  5  f°r  the  latter  it  is  three  times  as  great.  Setting  aside  air, 
which  is  a  mixture  and  which  contains  more  easily  liquefiable 
oxygen,  the  coefficients  for  constant  volume  of  H2,  N2,  and  CO 
are  identical. 

Finally,  for  hydrogen  the  coefficient  of  expansion  does  not  vary 
appreciably  with  the  pressure. 

The  inequality  of  the  coefficients  of  expansion,  however,  does 
not  prevent  us  from  taking  any  gas  whatever  to  define  the  scale 
of  temperature,  provided  we  apply  to  it  the  proper  coefficient 
determined  by  experiment  between  o°  and  100°.  The  scales  are 
identical,  if  the  coefficients  of  expansion  do  not  vary  with  the 
temperature.  This  is  the  conclusion  to  which  Regnault  came 
from  a  comparison  of  thermometers  at  constant  volume,  differing 
by  their  initial  pressure  or  the  nature  of  the  gas.  Here  are  the 
results  obtained,  starting  from  the  fixed  points  o°  and  100°,  by 
the  aid  of  the  following  formulae: 

pv  =  nRT, 
pQv  =  nRT0, 


J_ 

ICO 


p-po 


r-  r0 
r,oo  -  rr 


AIR  THERMOMETER. 


P°=75i  mm. 

p°=i486  mm. 

Degrees. 

Degrees. 

156.18 

156.19 

2S9-50 

259-4I 

324-33 

324.20 

PRESSURE  =  760  MILLIMETERS. 


Air  thermometer. 

Hydrogen 
thermometer. 

Air  thermometer. 

CO2  thermometer. 

Degrees. 

Degrees. 

Degrees. 

Degrees. 

141-75 

141.91 

I59-78 

l6o.OO 

228.87 

228.88 

267.35 

267.45 

325-40 

325.21 

322.8 

322.9 

20  HIGH  TEMPERATURES 

The  deviations  do  not  exceed  0.2°,  a  value  that  Regnault 
estimated  not  to  exceed  the  limits  of  error  of  his  experiments;  he 
concluded  from  this  that  one  gas  may  be  used  as  well  as  another, 
and  he  took  air  for  the  normal  thermometer. 

Nevertheless  his  experiments  on  sulphurous  acid  had  shown 
a  very  marked  variation  of  the  coefficient  of  expansion  of  this 
gas  with  the  temperature.  The  following  table  gives  the  mean 
coefficient  at  constant  volume  between  o°  and  t°  for  this  case : 

0 
98.0 0.0038251 

102 .45 ' 38225 

185-42 37999 

257.17 37923 

299.90 37913 

310.31 37893 

By  analogy  it  is  permissible  to  suppose  that  a  similar  effect 
ohould  take  place  with  the  other  gases;  but  the  differences  were 
then  too  small,  and  the  degree  of  precision  of  the  methods  of 
Regnault  insufficient  to  detect  it. 

Results  Obtained  by  Chappuis.  —  This  effect  has  been  demon- 
strated by  experiments  of  very  great  precision  made  at  the 
Bureau  International  des  Poids  et  Mesures,  at  Sevres.  Chappuis 
has  found,  between  o°  and  100°,  systematic  deviations  between 
thermometers  of  hydrogen,  nitrogen,  and  carbonic  acid,  filled  at 
o°  under  a  pressure  of  1000  mm.  of  mercury. 

Hydrogen  ther.  N  ther.-H  ther.         N  ther.-COj  then 

-    15°  -0.016°  -0.094° 

oo  o 

+  25  +O.OU  +0.050 

+  5°  +0.009  +0.059 

+  75  +O.OH  +0.038 

+  100  o                              o 

In  this  table,  taking  as  definition  of  the  temperature  the 
hydrogen  thermometer  at  constant  volume,  the  numbers  in 
the  last  two  columns  indicate  the  deviations  observed  with  the 
thermometers  of  nitrogen  and  carbonic  acid;  it  is  certain  that 
these  deviations  are  systematic.  These  results  allow  of  the 
determination  of  the  mean  coefficients  of  expansion: 


STANDARD    SCALE   OF   TEMPERATURES 


21 


t  0  (hydrogen) 

o 

ioo          0.00366254 


/3  (nitrogen)  /8  (carbonic  acid) 

0.00367698  0.00373538 

367466  372477 


Thus  the  coefficients  decrease  with  rise  of  temperature,  while 
remaining  higher  than  that  of  hydrogen,  to  which  they  tend 
to  approach.  The  more  recent  work  of  Chappuis  and  Harker 
and  others  in  the  establishment  of  a  normal  scale  of  tempera- 
tures for  high  temperatures  will  be  discussed  in  the  following 
sections. 

In  the  interval  o°  to  100°,  the  values  given  above,  calculated 
from  Chappuis'  data  of  1888,  may  not  be  absolutely  exact,  but 
they  are  probably  very  nearly  correct.  Some  of  the  later  results 
are  given  below;  those  marked  Callendar  are  calculated  by  him 
from  the  data  of  Kelvin  and  Joule,  using  a  modified  formula; 
Chappuis'  results  are  from  his  latest  determinations  (1902),  while 
those  of  Lehrfeldt  and  Rose-Innes  are  calculations  involving 
special  thermodynamical  assumptions. 


DIFFERENCE  BETWEEN  SCALES  OF  NITROGEN  AND  HYDRO- 
GEN THERMOMETERS. 


~  tfa  v°l-  =  const., 


100  cms. 


Temp.  Cent. 

Callendar. 
1903. 

Chappuis. 
1902. 

Rose-Innes. 
1901. 

Lehrfeldt. 
1898. 

+  20 

+  .006 

+  .005 

+  .002 

+  .011 

+40 

+  .009 

+  .008 

+  .002 

+  .017 

+50 

+  .009 

-j-.oio 

+  .002 

+  .OIQ 

+60 

+  .008 

+  .009 

+  .002 

+  .019 

-I-  80 

+  .005 

+  .004 

+  .001 

+  .015 

Normal  Scale  of  Temperatures.  —  It  results  from  these  ex- 
periments that  the  different  scales  furnished  by  the  various 
gas  thermometers  are  not  rigorously  identical;  the  deviations 
between  o°  and  100°  are  very  small,  but  their  existence  is  certain. 
It  becomes  necessary,  therefore,  in  order  to  have  a  scale  of  tem- 
perature rigorously  defined,  to  make  a  choice  of  the  nature  of  the 
gas,  of  its  manner  of  dilatation,  and  of  its  initial  pressure. 


22  HIGH  TEMPERATURES 

The  normal  thermometer  selected  by  the  Bureau  International 
des  Poids  et  Mesures  to  define  the  practical  scale  of  temperatures, 
and  everywhere  adopted  to-day,  is  the  hydrogen  thermometer, 
operated  at  constant  volume  and  filled  with  gas  at  1000  milli- 
meters of  mercury  at  the  temperature  of  melting  ice. 

For  high  temperatures  this  definition  is  inadmissible,  because 
we  would  reach  such  pressures  that  the  apparatus  could  not  with- 
stand. The  use  of  the  method  at  constant  volume,  that  is  to 
say,  at  invariable  mass,  is  besides  bad  on  account  of  the  per- 
meability of  the  coverings  at  high  temperatures.  It  would  be  of 
great  advantage  to  be  able  to  employ  a  gas  other  than  hydrogen 
and  operate  the  thermometer  at  variable  mass.  Practically,  it 
has  been  the  custom,  in  most  of  the  recent  work  at  high  tem- 
peratures, to  use  nitrogen  gas  at  reduced  pressure,  150  to  300 
mm.  of  Hg  at  o°  C.,  although  there  has  been,  as  yet,  no  formal 
agreement  as  to  the  gas  or  type  of  thermometer  to  use  in  defining 
the  high  temperature  scale. 

In  the  actual  state  of  experimentation  at  high  temperatures,  it 
has  been  impossible  as  yet  to  obtain  results  exact  to  better  than 
i°,  and  indeed,  practically,  we  are  far  from  arriving  at  this 
accuracy  for  the  highest  temperatures  measureable.  It  is  very 
likely  that  we  can,  under  these  conditions,  employ  indifferently 
for  the  construction  of  the  normal  thermometer  any  permanent 
gas  whatsoever  that  does  not  diffuse  into  or  through  the  contain- 
ing bulb.  According  to  the  preceding  experiments,  all  the  gases 
would  have  a  dilatation  slightly  greater  than  that  for  hydrogen, 
and  their  coefficient  of  expansion,  which  decreases  with  rise  of 
temperature,  would  approach  that  for  hydrogen.  For  deter- 
mining experimentally  the  error  possible  with  a  normal  thermom- 
eter thus  modified,  we  possess  the  following  experimental  data. 

Crafts  compared  in  the  neighborhood  of  1500°  the  expansion 
at  constant  pressure  of  nitrogen  and  carbonic  acid,  and  found  for 
this  latter  the  mean  coefficient  0.00368  in  assuming  0.00367  for 
nitrogen. 

The  experiments  were  made  by  displacing  in  a  Meyer's  tube 
nitrogen  by  carbonic  acid,  or  carbonic  acid  by  nitrogen. 


STANDARD   SCALE  OF  TEMPERATURES  23 

10  cc.  N2  displace  10  cc.  CC>2  displace 

io.o3ofCO2  9-95ofN2 

10.01  9-91 

10.00  9-98 

10.03  9-93 

9-95 

10.09  Mean    9.94 


Mean  10.02 

The  two  measurements  give  positive  and  negative  differences 
of  the  same  order  of  magnitude;  but  it  should  be  noticed  that 
the  observed  deviation  (^innr  on  an  average)  hardly  exceeds 
the  possible  error  of  observation.  However  it  may  be,  carbonic 
acid,  which  differs  much  from  the  permanent  gases  at  ordinary 
temperatures,  no  longer  so  differs  in  an  appreciable  degree  at 
1500°. 

Violle  made  some  comparative  measurements  on  the  air 
pyrometer  used  at  constant  pressure  and  constant  volume  in  his 
determinations  of  the  specific  heat  of  platinum. 

Volume  constant.  Press,  constant.  Difference.         .     , 
1171°        .                   1165°  6° 

1169  II66  3 

H95  "92  3 

There  was  on  an  average  a  deviation  of  only  4°  between  the 
two  modes  of  observation,  and  the  greater  part  of  this  deviation 
should  be  laid  to  accidental  variations  of  the  gaseous  mass 
resulting  from  the  permeability  of  the  coverings. 

Chappuis  has  made  an  exhaustive  experimental  study  of  the 
divergences  of  gases  from  the  normal  scale  at  comparatively  low 
temperatures  and  he  finds  that  the  coefficient  of  nitrogen  (at 
constant  volume)  gradually  diminishes  as  above  stated  (p.  21), 
but  that  at  about  75°  C.  it  reaches  a  limiting  value  equal  to 


=  0.00367330, 

and  it  may  be  assumed  that  above  this  temperature  the  gas  is 
in  a  perfect  state. 

The  mean  coefficient  at  constant  volume  for  this  gas  between 
o°  and  1  00°  is 

180-100  =  0.00367466 


24  HIGH  TEMPERATURES 

and  the  limiting  value  for  an  initial  pressure  P0  =  o  is 
/3Po=0  =  0.0036617. 

This  follows  from  the  divergence  that  Chappuis  and  Harker 
found  for  the  constant-volume  nitrogen-thermometer  from  the 
normal  scale  of  temperatures,  in  terms  of  the  initial  pressure; 
their  experiments  gave 

aft 

—  =  i.28.io~8  per  mm.  change  in  pressure. 

dp 

It  is  to  be  remembered  that  when  the  volume  or  pressure  coeffi- 
cient is  found  for  any  pressure,  that  value  is,  by  definition,  the 
one  to  use  in  computing  the  normal  scale  of  temperatures. 

The  experiments  of  Chappuis  and  Harker  were  carried  out 
at  the  International  Bureau  of  Weights  and  Measures  and  in- 
cluded also  a  comparison  of  the  platinum-resistance  and  nitrogen 
thermometers  up  to  500°  C.  and  a  determination  of  the  sulphur 
boiling  point,  to  which  questions  we  shall  return. 

Such  a  normal  scale  of  temperature  for  the  nitrogen  thermom- 
eter is  given  by  finding  the  coefficient  0,  at  o°  C.  for  a  pressure 
PQ  which  the  gas  would  have,  supposing  it  to  remain  perfect  in 
the  range  o°  to  100°.  If  P0  =  100  cm.,  PIOO  =  136.7466  cm.; 

whence    JP0'  =  100.0086    and    0  =  Pm  ~Tf/°  =  0.00367348,  if 

100  P0 

Aim  =  0.00367330  as  stated  above. 

Nitrogen  at  constant  pressure  gives,  according  to  Chappuis, 

8a 

—  =  1.19.10  8  per  mm. 

dp 

and  .  ap=Q  =  0.0036612. 

The  divergences  from  the  normal  scale  for  this  gas  are  about 
double  those  at  constant  volume,  and  the  divergences  between 
the  uncorrected  scale  and  the  theoretical  scale  of  the  constant- 
volume  thermometer,  whose  constants  are  given  above  and  which 
represents  the  normal  scale  of  temperatures,  are  proportional 
to  the  temperature  measured  from  100°  and  have  the  following 
values : 


STANDARD   SCALE  OF  TEMPERATURES 


At  100°. 

200     . 

300  . 

400  . 


o.ooo 

.023 

.047 
.070 


These  deviations  are  evidently  very  slight  and  are  entirely 
negligible  within  this  range  for  practically  all  pyrometric  uses. 
We  shall  see,  however,  that  at  1000°  this  correction  may  assume 
a  certain  importance. 

For  hydrogen,  the  limiting  values  given  by  D.  Berthelot  are: 

/3P=0  =  0.0036625, 
ap=0  =  0.0036624, 

and  the  deviations  of  this  gas  from  the  normal  scale  are  imma- 
terial. 

The  latest  results  of  Chappuis  on  the  elastic  properties  of  the 
various  thermometric  gases  are  given  in  the  following  table: 


EXPANSION  COEFFICIENTS  OF  THERMOMETRIC  GASES 
ACCORDING  TO   CHAPPUIS. 


io«X 

Hydrogen. 

Nitrogen. 

Air. 

CO,.     • 

367?  o 

3733    ^ 

00      40 

•2671;   4 

372O    O 

3662    <K 

3674  6 

3674  4i 

3726    2 

3662    tJe 

3661    7 

367O 

5/3/6/V.                

O 

o  0128 

Qfo     20-                          

3677  o 

376O    2 

CtQ     4fl.                    

3674   07 

37^3    6 

<*0—  100  

«p=o  
Sa/8Po  

3660.04 
3662.49 
0.0186 

.        3673-I5 
3661.2 
O.OII9 

3672.82 

3741-0 
3671 

Where 


i    dV 


i     dP        ,  _ 
=  ""  *        and  ^°  =  I00°  mm- 


Jaquerod  and  Perrot  have  compared  the  coefficients  of  expan- 
sion j8  of  several  gases  in  a  silica  bulb  between  o°  C.  and  the  melt- 
ing point  of  gold  and  at  initial  pressures  from  170  to  230  mm. 
of  Hg,  with  the  following  results: 


26  HIGH  TEMPERATURES 

EXPANSION  COEFFICIENTS  AT  HIGH  TEMPERATURES. 


Gas. 

ft. 

Melting  point 
of  Au. 

Pressure  at 
o°  C.  mm. 

Nitrogen  

o  0036643 

1067.2 

200-230 

Oxygen  .  . 

0.0036652 

1067.5 

180-230 

Air 

o  0036663 

1067    2 

230 

CO  

o  0036638 

1067   Ofi 

240 

f^d  (  p  =  240  

o  00^67^6 

M  p=  170 

o  0036713 

|     1066.5 

170 

We  can  then  affirm  that,  in  employing  any  permanent  gas 
with  any  mode  of  dilatation,  we  shall  not  differ  certainly  by  more 
than  i°  at  1000°  from  the  temperature  of  the  normal  scale,  and 
that,  with  the  exception  of  COa,  all  the  permanent  gases  have 
very  nearly  the  same  expansion  coefficient.. 

Theoretically  it  would  be  preferable  to  use  hydrogen  under 
reduced  pressure,  which  would  certainly  not  give  deviations  of 
i°  from  the  normal  scale;  but  there  is  always  the  danger  of  the 
passage  of  this  gas  through  the  coverings  and  of  its  combustion 
by  oxygen  or  oxides. 

Practically  it  would  be  better  to  take  nitrogen,  whose  expan- 
sion deviates  little  from  that  of  hydrogen,  less  than  the  deviation 
of  air.  Callendar  has  suggested  the  use  of  helium  or  one  of  the 
other  newly  discovered  inert,  monatomic  gases,  such  as  argon, 
as  they  diverge  less  than  nitrogen  from  the  hydrogen  scale,  can- 
not dissociate  and  do  not  pass  through  metals,  at  least  in  the 
case  of  argon. 

For  high  temperatures  the  normal  thermometer  will  be,  then, 
one  of  nitrogen  or  other  inert  gas. 

Thermodynamic  Scale.  —  It  is  defined,  in  terms  of  Carnot's 
principle  applied  to  a  reversible  cycle  working  between  two 
sources  at  constant  temperatures,  by  the  relation 


-  =  —  fl) 

Co      TV 

i .  A pproximate  Expression. — Consider  Carnot's  cycle  formed, 
as  is  well  known,  of  two  isotherms  and  two  adiabatics,  and  let  us 
seek  the  quantity  of  heat  absorbed  following  the  isotherm  7\. 


STANDARD    SCALE   OF   TEMPERATURES  27 

From  Joule's  experiments  we  have  approximately 

Ql  =  AJ*p  dv. 
The  laws  of  Mariotte  and  Gay-Lussac  give 


where  /  is  the  temperature  of  the  gas  thermometer;  then, 


and       Q. 

Similarly, 


Equation  (i)  becomes 


But  the  experiments  on  adiabatic  expansion  give 
piff  =  const., 

where  7  is  the  ratio  of  the  specific  heats  at  constant  pressure 
and  volume,   and  combining  with  the  laws   of  Mariotte  and 

Gay-Lussac, 

py-i  .  t~y  =  const. 

Consequently  —  depends  only  on  the  ratio  -  ,  which  is  the  same 
po  A> 

the  whole  length  of  the  two  isotherms.     Thus 

£-C 

Po'      Po" 

.  /        .  / 
PI         po 

or  7"  =  3T^* 

pi        Po 


2g  HIGH  TEMPERATURES 

Equation  (2)  then  takes  the  very  simple  form 


that  is  to  say,  the  ratio  of  the  absolute  thermodynamic  temperatures 
is  equal  to  the  ratio  of  the  absolute  temperatures  of  the  gas  thermome- 
ter; and  if  on  the  two  scales  it  is  agreed  to  take  equal  to  100  the 
interval  comprised  between  the  temperatures  of  melting  ice  and 
the  vapor  of  boiling  water,  we  have,  at  any  temperature,  the 
equality 


But  this  is  only  a  first  approximation,  for  we  have  employed 
relations  that  are  but  roughly  so:  the  laws  of  Joule,  Mariotte, 
and  Gay-Lussac. 

2.  Second  Approximation.  —  Reconsider   the  problem  by  a 

more  exact  method.     Since  T  differs  very  little  from  -  >  and 

.  «  +  t 

since  the  laws  of  Mariotte  and  Gay-Lussac  are  nearly  true,  we 
write,  following  a  method  of  calculation  indicated  by  Callendar, 


</>  being  a  very  small  function  of  p  and  of  T  (thermodynamic 
temperature). 

We  have,  then,  between  the  temperature  of  the  gas  thermom- 
eter and  the  thermodynamic  temperature,  the  relation 


which  will  permit  of  passing  from  one  scale  of  temperature  to 
the  other  if  we  know  the  corresponding  value  of  <£. 

Consider,  as  before,  Carnot's  cycle,  and  let  us  determine  the 
heat  of  isothermal  expansion  in  a  more  exact  manner,  by  utilizing 


STANDARD   SCALE  OF  TEMPERATURES  29 

the  experiments  of  Joule  and  Thomson  on  the  expansion  through 
a  porous  plug,  and  those  of  Regnault  on  the  deviations  from 
Mariotte's  law. 

We  write  for  this  that  the  changes  in  energy  between  two  given 
isothermal  states  are  the  same,  either  for  the  reversible  expansion 
or  for  the  expansion  of  Joule  and  Thomson. 


€  being  the  very  feeble  change  in  heat  of  the  gas  accompanying 
its  passage  through  the  porous  plug,  in  the  experiment  of  Joule 
and  Thomson.  We  get  from  this 

ft  =  A  I      v  dp  +  /  -^  dp  (at  constant  temperature),    (3) 
Jtf  J  dp 

for  d  (pv)  =  p  dv  +  v  dp. 

The  relation  pv  =  RT  (i  -  0) 

gives  for  the  value  of  v 

RT  (        ^ 
»=  —  (i  -<*>)> 

which,  substituted  in  equation  (3),  leads  to 


Similarly,  we  have 


If  we  introduce  these  values  in  the  expression  for  Carnot's 
cycle,  after  division  by  TI  and  TQ  we  should  find  an  identity: 


- 
p       YI   dp 

--i-£-° 

p       TV  dp 


>  HIGH  TEMPERATURES 

The  law  of  adiabatic  expansion  gives 

^7t77  =  J>         lo* 
pipo 


=  o. 


In  order,  then,  that  the  expression  reduce  to  an  identity  it  is 
necessary  that 


Referring  to  the  experiments  on  air  of  Joule  and  Thomson,  we 
have 

<t>  =  0.001173-  £ 

A> 

po  being  the  atmospheric  pressure,  and  T0  the  temperature  of 
melting  ice. 

This  is  still  an  approximate  result,  for  we  have  depended  upon 
the  experiments  of  Joule  and  Thomson  and  on  the  law  of  adiabatic 
expansion;  however,  the  approximation  is  more  close.  If  it 
seems  sufficient  for  air,  it  is  certainly  not  so  for  carbonic  acid. 
Neither  is  the  formula  rigorously  exact  for  air. 

Gas  Scale  Corrections.  —  Callendar  has  calculated  the  correction 
to  make  to  the  air  thermometer  readings  by  extrapolation  up  to 
1000°,  and  he  found  the  following  results: 


Readings  of 

Volume  constant. 

Pressure  constant. 

thermometer. 

<A 

At 

<t> 

A/ 

"     0° 

O.OOII73 

0 

0.001173 

0 

100 

0.000627 

O 

0.000457 

0 

2OO 

393 

0.04 

225 

0.084 

300 

267 

0.09 

127 

O.2O 

500 

H7 

0.23 

52 

0.47 

1000 

54 

O.62 

12 

1.19 

The  deviations  of  the  air  thermometer  at  high  temperatures 
are  thus  very  slight  if  concordance  is  established  at  o°  and  100°, 
and  we  have  seen  that  in  the  case  of  nitrogen  the  experiments 
of  Chappuis  and  Harker  have  shown  the  same  to  be  true  for 
this  gas. 


STANDARD   SCALE   OF  TEMPERATURES  31 

Callendar,  in  a  more  recent  computation  based  upon  the  work 
of  Kelvin  and  Joule  and  the  experiments  of  Chappuis  and  others, 
arrives  at  the  following  values  for  the  scale  corrections  for  the 
best  thermometric  gases: 

SCALE  CORRECTIONS  FOR  GASES,  ASSUMING  80  =  273.10°. 


Constant  pressure,  76  cm. 

Constant  volume,  p1  =  ioo  cm. 

centigrade. 

Helium. 

Hydro- 

Nitro- 

Air. 

Helium. 

Hydro- 

Nitro- 

Air. 

gen. 

gen. 

gen. 

gen. 

-    ISO 

+0.073 

+0.084 

+0-945 

+0.901 

—  O.O26 

+0.013 

+0.195 

+0.186 

—     IOO 

+    .030 

+    .022 

+    .328 

+    -3H 

—     .OI2 

+    -005 

+    .080 

+    .076 

-    50 

•f-    .009+    .006 

+    .090 

+    .086 

—     .004 

+    .OO2 

+    .024 

+    -023 

—     20 

+     .003+     .002 

+    .025 

+    .024 

—     .OOI 

+    .OOO 

+    -007 

+    -007 

+       20 

—  .OOl6 

—  .OOOQ 

—  .0141 

—  .0134 

+  .0008 

—  .  0003 

—  .0043 

+  .0041 

+       40 

—  .0022 

—  .0013 

-.0195 

-  .0186 

+  .0011 

—  .0004 

-.0059 

+  .0056 

'       +       50 

—  .0022 

—  .0013 

—  .0195 

-  .0186 

+  .0011 

—  .0004 

-.0059 

+  .0056 

+       60 

—  .0021 

—  .0012 

—  .0180 

—  .0172 

+  .0011 

—  .0004 

-  .0054 

+  .0053 

+       80 

-  .0013 

—.0008 

—  .0113 

—  .0108 

+  .0007 

—  .0002 

-.0038 

+  .0034 

+    15° 

+  .0054 

+  .0029 

+    -043 

+    .041 

—  .0031 

+  .0010 

+  .0143 

+  .0136 

+    200 

+  .0128 

+  .0068 

+     .101 

+    .096 

—  .0076 

+  .0024 

+  .035 

+  •033 

+    300 

+  .0332 

+  .0165 

+    -243 

+    -232 

-.203 

+  .0059 

+  .088 

+  .084 

+    450 

+  .071 

+  .034 

+    -495 

+    .472 

—  .047 

+  .013 

+  .189 

+  .180 

+  IOOO 

+  •243 

+  .104 

+  1-53 

+  1.46 

-.187 

+  .044 

+  .646 

+  .616 

The  above  table  indicates  that  for  the  gases  hydrogen  and 
helium  no  attention  need  be  paid  to  the  thermodynamic  correc- 
tion, for  it  is  quite  negligible  for  the  whole  temperature  range  for 
these  two  gases.  All  the  gases  are  also  seen  to  have  a  greater 
correction  at  constant  pressure  than  at  constant  volume.  Again 
it  is  to  be  noted  that  at  small  initial  pressures  these  corrections 
will  be  proportionally  reduced,  and  finally  that  it  is  only  in  the 
most  refined  work  that  this  correction  need  be  applied,  as  in  the 
establishment  of  a  fixed  point  in  pyrometry  as  the  gold  fusing- 
point. 

D.  Berthelot  has  indicated  a  simple  method  for  calculating 
this  thermodynamic  correction  for  any  gas. 

For  a  constant-  volume  thermometer: 


373  273 


32  HIGH  TEMPERATURES 

T0  being  the  absolute  temperature  of  melting  ice  (273.10°),  T 
the  absolute  temperature  sought  corresponding  to  the  centigrade 
temperature  /  given  by  the  gas  thermometer  in  question  at  an 
initial  pressure  of  one  atmosphere.  For  other  pressures  p  the 

correction  to  t  must  be  multiplied  by  •*-  • 

For  the  constant-pressure  thermometer 

T  -  TQ  =  t\i- 


The  value  of  a  depends  upon  the  critical  constants  of  the  gas 
and  is  27     2    Tc3 

=  64R  '77' 

where  R  is  the  gas  constant  (here  — — Y  Tc  and  pc  the  critical 
pressure  and  temperature  respectively. 

TABLE  OF   CRITICAL  CONSTANTS. 


PC 

(. 

a 

Carbonic  acid                   

72  .9  atm. 

+  31.3 

2.188 

Oxygen                       

50.0 

-118 

0.422 

Air                '.  

39.0 

—  140 

•342 

Carbon  monoxide                              .  . 

•3.e   Q 

—  141 

.363 

Nitrogen  .                                  

33  6 

—  146 

•343 

Hydrogen                     

13  o 

—  240 

.016 

Helium                

3 

-268 

.009 

The  formulae  of  Berthelot  give  practically  identical  values  for 
the  thermodynamic  corrections  as  found  by  Callendar.  Buck- 
ingham has  discussed  in  detail  the  departures  of  the  temperature 
scales,  both  constant  volume  and  constant  pressure,  given  by  the 
several  gases,  from  the  thermodynamic  scale  by  a  method  similar 
to  that  of  Berthelot 's,  but  using  a  somewhat  simpler  equation 
of  state.  The  most  interesting  results  relate  to  the  behavior  of 
nitrogen,  which  is  now  generally  used  as  the  thermometric  gas 
in  high  temperature  measurements,  and  in  Fig.  i  are  given  the 
corrections  of  the  nitrogen  thermometer  at  PQ  =  1000  mm.  of 
Hg  taken  from  Buckingham's  paper. 


STANDARD   SCALE  OF  TEMPERATURES 


34  HIGH  TEMPERATURES 

It  should  be  noted  that  the  calculated  corrections  to  reduce 
the  readings  of  any  gas  thermometer  to  the  thermodynamic  scale 
are  extrapolations  from  data  on  the  Joule-Thomson  effect  made 
at  ordinary  temperatures.  This  is  probably  not  a  serious  source 
of  concern,  however,  as  both  Buckingham  and  Berthelot  show 
that  the  several  gases,  when  treated  by  the  method  of  corre- 
sponding states,  that  is  reduced  in  pressure  and  temperature  to 
the  fractions  of  their  critical  constants,  furnish  data  all  lying  on 
the  same  curve. 

Experimental  science  has  now  reached  such  a  development 
that  as  above  stated  these  corrections  to  the  thermodynamic 
scale  cannot  always  be  neglected. 

The  Ice  Point.  —  The  experiments  of  Kelvin  and  Joule  may 
also  be  used  to  determine  the  absolute  temperature  of  the  point 
of  fusion  of  ice  on  the  thermodynamic  scale.  Below  are  the 
results  of  a  computation  by  Lehrfeldt  made  several  years  ago. 

Gas-ther.  Thermodyn.  ther. 

Hydrogen  .................   273.08°  272.8° 

Air  .......................   272.43  273.27 

Nitrogen  .................   273  .  13  273  .  2 


Carbonic  acid  ........  ....   268.47       I  2?4'8i 

(  273.48  (Natanson) 

The  thermodynamic  temperature  of  melting  ice  should  be  in  all 
cases  the  same;  the  deviations  come  mainly  from  the  uncertainties 
in  the  measurements  of  the  heat  of  expansion,  indicating  the 
desirability  of  repeating  Joule  and  Thomson's  work  with  modern 
appliances. 

There  have  been  several  more  recent  computations  of  the 
temperature    of   fusion   of   ice    on    the    thermodynamic    scale 

(  =  -=-)  based  on  the  experimentally  found    deviations  of 

several  of  the  real  gases  from  the  ideal  state,  account  being  taken 
of  the  Joule-Thomson  effect  as  measured  by  various  observers, 
the  thermal  expansion  and  the  compressibility  as  determined 
by  Chappuis  and  by  Amagat,  the  computations  requiring  the 
use  of  a  modified  form  of  Van  der  Waal's  equation  of  state. 
Some  of  these  calculations  are  as  follows: 


STANDARD   SCALE  OF  TEMPERATURES 


35 


THERMODYNAMIC  TEMPERATURE  OF  MELTING  ICE  (00) 

Author.  Gases  used  in  com- 

putation. 0 


D.  Berthelot  (1903) H,  CO2,  Air 

Buckingham  (1907) H,  N,  CO2,  Air 

Rose-Innes  (1908) 


273.11 
273.174 
273.131 
273-I36 

The  following  table  gives  Callendar's  resume  of  the  expansive 
properties  of  the  thermometric  gases.  In  the  table  00  is  the  ther- 
modynamic  temperature  of  the  ice-point  as  determined  from 
hydrogen,  and  T0  this  point  on  the  various  gas  scales. 

EXPANSION  AND  PRESSURE  COEFFICIENTS  FOR  00  =  273.10°. 


Gas. 

Constant  pressure,  76  cm. 

Constant  volume,  p0—ioocm. 

e0-T0 

To 

i/r, 

e0-T0 

r« 

i/r, 

Helium 

-f-o.  10 

-  -135 

+  .70 
+  .71 

273.00 

273.235 
272.40 
272.39 

.0036628 
.00365985 
.0036708 
.0036709 

+0.19 
+    .067 
+    -99 
+    -96 

272.91 

273-034 
272.  ii 
272.14 

.0036640 
.00366254 
.00367466 
.00367425 

Hydrogen    .   .  . 

Nitrogen  
Air  

Chappuis' latest  values  give  in  the  case  of  hydrogen-  =  273.038 

a 

and  -  =  273.033   for  zero  pressure   on  the  hydrogen  scale,  as 

computed  by  himself,  showing  no  sensible  difference  in  the  two 
hydrogen  scales  in  the  range  o°  to  100°  C,  and,  taken  with  the 
preceding  tables,  that  the  hydrogen  and  thermodynamic  scales 
differ  by  about  0.10°  C.  at  o°  C.  Our  knowledge  of  the  thermo- 
dynamic scale,  as  realized  by  correcting  the  several  gas  scales, 
may  be  said  to  be  in  a  very  satisfactory  condition.  As  we  shall 
see  in  the  chapter  on  the  laws  of  radiation,  the  normal  or  thermo- 
dynamic scale  of  temperatures  may  be  extended  to  temperatures 
indefinitely  high  in  terms  of  the  intensity  of  radiation  total  or 
monochromatic,  which  proceeds  from  a  small  opening  in  any 
enclosure  at  constant  temperature  throughout.  We  shall  have 
realized,  therefore,  a  single  standard  or  normal  temperature  scale 


36       '  HIGH  TEMPERATURES 

independent  of  the  properties  of  any  particular  substance,  con- 
tinuous from  the  absolute  zero  to  the  highest  temperatures  that 
may  be  produced,  and  one  that  is  practically  reproducible  for  all 
technical  and  scientific  purposes,  by  methods  that  are  available 
in  the  several  standardizing  laboratories. 


CHAPTER  II. 
GAS  PYROMETER. 

Introduction.  —  We  have  seen  that  the  standard  scale  of  tem- 
peratures adopted  by  the  International  Committee  of  Weights 
and  Measures  is  given  by  a  certain  constant-volume  hydrogen 
thermometer,  namely  that  of  the  International  Bureau  at  Sevres, 
which  instrument,  however,  has  not  been  used  to  measure  tem- 
peratures above  100°  C.  The  type  of  gas  thermometer  which  is 
to  be  considered  standard  for  higher  temperatures  has  not  as  yet 
been  agreed  upon  by  any  authoritative  body,  but  for  reasons 
which  we  shall  develop,  the  constant-volume  nitrogen  thermome- 
ter appears  to  have  the  preference,  at  least  for  temperatures 
above  200°  C.  From  what  we  have  seen  in  the  preceding 
chapter,  it  is  practically  immaterial  in  the  definition  of  the  high 
temperature  scale  what  form  of  thermometer  is  actually  used, 
as  the  indications  of  any  of  the  gas  thermometers  may  readily 
be  compared  with  those  of  another  by  well  established  methods 
of  computation  and  reduced  with  great  accuracy  to  a  common 
theoretical  basis,  that  of  the  thermodynamic  scale. 

It  may  be  well  to  recall,  at  this  point,  in  what  consists  the 
actual  operation  of  the  location  of  a  temperature  on  the  chosen 
gas  scale,  and  point  out,  at  the  same  time,  some  of  the  difficulties 
involved.  The  gas  thermometer  bulb  must  be  brought  through- 
out its  volume  to  a  sufficiently  uniform  temperature.  To  obtain 
a  volume  of  500  c.c.  of  gas,  for  example,  constant  in  temperature 
to  i°  at  1000°  C.  has  not  yet  been  attempted  by  any  experimenter. 
Whatever  the  system  of  gas  thermometry  used,  on  account  of 
the  transient  nature  of  the  phenomenon  measured,  pressure  on 
a  manometer,  a  mass  of  displaced  mercury,  etc.,  it  is  also  neces- 
sary, except  in  certain  special  cases  as  some  boiling  points,  to 
bring  to  this  same  temperature  some  other  body  whose  registra- 

37 


38  ,  HIGH  TEMPERATURES 

tions  are  more  permanent,  such  as  a  mercury,  platinum  resist- 
ance, or  thermoelectric  thermometer,  or  rarely  a  metal  at  its 
melting  point,  and  finally  it  is  practically  necessary  to  transfer 
the  readings  of  the  gas  thermometer  by  means  of  this  auxiliary 
thermometer  to  a  series  of  fixed  temperatures  such  as  freezing 
and  boiling  points.  The  gas  scale,  therefore,  is  found  in  practice 
to  be  finally  a  discontinuous  one,  or  at  best  represented  by  con- 
tinuous interpolations  in  terms  of  some  empirical  law,  not  the 
gas  law.  We  shall  see  that  there  are  further  and  very  serious 
limitations  in  the  attainment  of  great  accuracy  with  the  gas  ther- 
mometer; thus,  the  space  containing  gas  between  the  hot  and 
cold  parts  is  at  an  unknown  average  temperature;  the  expan- 
sion of  the  bulb  with  heat  must  be  corrected  for;  and  the  bulb 
must  be  of  sufficient  rigidity  and  impermeability  at  the  highest 
temperatures. 

The  gas  thermometer,  as  we  have  seen  above,  need  not  of 
necessity  be  used  for  the  measurement  of  temperatures;  it  suffices 
to  make  use  of  it  for  the  standardization  of  the  different  processes 
employed  in  the  determination  of  temperatures,  but  a  priori 
there  are  not  on  the  other  hand  any  absolute  reasons  for  discard- 
ing it  in  cases  other  than  these  standardizations.  Indeed,  it  has 
often  been  so  employed,  although,  as  we  shall  see,  it  is  usually 
more  convenient  to  make  use  of  some  other  method  in  practice. 

We  shall  describe  first  the  standard  gas  thermometer,  and  then 
discuss  in  considerable  detail  the  factors  that  enter  into  the 
construction  and  theory  of  gas  thermometers  suitable  for  high 
temperatures,  and  give  an  account  of  several  of  the  various 
investigations  in  gas  thermometry,  and  finally  call  attention  to 
the  requirements  for  future  work  in  this  domain. 

Standard  Gas  Thermometer.  —  This  thermometer,  that  of 
the  International  Bureau  of  Weights  and  Measures  at  Sevres, 
France,  is  a  constant-volume  thermometer  filled  with  pure,  dry 
hydrogen,  under  the  pressure  of  i  m.  of  mercury  at  the  tempera- 
ture of  melting  ice.  It  consists  of  two  essential  parts:  the  bulb, 
enclosing  the  invariable  gaseous  mass,  and  the  manometer,  serv- 
.ing  to  measure  the  pressure  of  this  gaseous  mass. 


GAS   PYROMETER 


39 


The  bulb  is  made  of  a  platinum-indium  tube  whose  volume  is 
1.03899  liters  at  the  temperature  of  melting  ice.  Its  length  is 
1. 10  m.,  and  its  outer  diameter  0.036  m.  It  is  attached  to  the 
manometer  by  a  capillary  tube  of  platinum  of  0.7  mm.  diameter. 


Fig.  2.     Mounting  of  Thermometer  Bulb. 

A  diameter  of  0.5  mm.  is  as  small  as  can  be  allowed  in  the 
colder  part  of  such  capillaries  on  account  of  the  lag  in  obtaining 
pressure  equilibrium. 

This  bulb  is  supported  horizontally  in  a  double  box  with 
interior  water  circulation.  For  the  determination  of  the  100° 
mark,  indispensable  for  standardization,  the  bulb  can  be  placed 
in  the  same  way  in  a  horizontal  heater  supplied  with  steam  and 
composed  of  several  concentric  coverings. 

Manometer.  —  The  manometric  apparatus  is  mounted  upon  an 
iron  support  of  2.10  m.  height,  which  is  made  of  a  railway  rail 
firmly  bolted  to  a  tripod  of  wrought  iron.  The  lateral  parts 
attached  to  this  rail,  planed  their  entire  length,  carry  sliding 
pieces  to  which  are  fastened  the  manometer  tubes  and  a  barom- 
eter. Fig.  3  represents,  in  a  slightly  modified  form,  the  mano- 
metric apparatus.  It  is  composed  essentially  of  a  manometer 
open  to  the  air  whose  open  arm  A  serves  as  cistern  for  a  barom- 
eter R.  The  other  arm  B,  closed  half-way  up  by  a  piece  of 
steel  H,  is  attached  to  the  thermometric  bulb  by  the  capillary 
tube  of  platinum  C.  The  two  manometer  tubes,  each  of  25  mm. 
interior  diameter,  have  their  lower  ends  fixed  into  a  block  of 
steel  S.  They  communicate  with  each  other  by  holes  of  5  mm. 


40  HIGH  TEMPERATURES 

diameter  bored  in  the  block.  A  stopcock  E  permits  closing  this 
connection.  A  second  three-way  cock  F  is  screwed  on  the  same 
block.  One  of  its  branches  can  serve  to  let  mercury  run  out;  the 
other,  to  which  is  attached  a  long  flexible  steel  tube,  puts  the 
manometer  in  communication  with  a  large  reservoir  of  mercury 


Fig.  3.    Manometer  of  Standard  Thermometer. 

D  which  can  be  raised  or  lowered  the  length  of  the  support, 
either  rapidly  by  hand,  or  slow-motioned  by  means  of  a  screw. 
The  barometer  which  sets  in  the  open  branch  is  fixed  at  its 
upper  part  on  a  carriage  G  whose  vertical  displacement  is  regu- 
lated throughout  a  length  of  0.70  m.  by  a  strong  screw.  The 
latter  is  held  at  its  two  ends  by  two  nuts  which  permit  it  to  turn 
without  longitudinal  motion;  it  works  in  a  screw  attached  to  the 


GAS  PYROMETER  41 

carriage,  and  carries  at  its  lower  end  a  toothed  pinion  which  works 
into  a  cogwheel.  It  suffices  to  turn  this  wheel  by  acting  upon 
the  rod  which  serves  as  axis  in  order  to  raise  or  lower  the  carriage 
with  the  barometer  tube.  This  last  has  a  diameter  of  25  mm. 
in  its  upper  part.  The  chamber  is  furnished  with  two  indexes 
of  black  glass  soldered  to  the  interior  of  the  tube  at  0.08  m.  and 
o.  1 6  m.  from  the  end.  The  points  of  these  indexes,  convex  down- 
wards, coincide  sensibly  with  the  axis  of  the  barometric  chamber. 
The  part  of  the  barometer  which  fits  into  the  open  manometer 
arm  has  a  diameter  greater  than  o.oi  m.,  and  ends  below  in  a 
narrower  tube  curved  upwards. 

The  piece  of  steel  which  ends  the  closed  arm  at  H  is  adjusted 
to  this  tube,  leaving  between  itself  and  the  tube  but  a  very  slight 
space,  which  is  filled  with  sealing  wax.  It  rests  upon  the  upper 
rim  of  this  tube,  to  which  it  is  besides  pressed  by  leather  washers 
tightly  screwed  up.  At  its  lower  end  it  terminates  in  a  perfectly 
smooth  polished  plane,  which  is  adjusted  to  be  horizontal.  In 
the  middle  of  this  surface,  near  to  the  opening  of  the  canal  which 
prolongs  the  joining  tube,  there  is  fixed  a  very  fine  platinum 
point,  whose  extremity,  meant  to  be  used  as  a  reference  mark,  is 
at  a  distance  of  about  0.6  mm.  from  the  plane  surface. 

Above  this  piece  is  a  tube  B  of  25  mm.  interior  diameter,  open 
above  and  connected  below  to  the  open  arm  of  the  manometer. 

Since  the  measurement  of  a  column  of  mercury  is  more  easily 
made  and  with  greater  precision  when  the  menisci  whose  dif- 
ference of  level  it  is  desired  to  find  are  situated  along  the  same 
vertical,  the  barometer  tube  R  is  bent  so  as  to  bring  into  the  same 
vertical  line  the  axis  of  the  closed  arm  of  the  manometer  and 
that  of  the  barometer.  Under  these  conditions,  the  communi- 
cation between  the  two  manometer  arms  A  being  established 
through  E,  the  total  pressure  of  the  gas  inclosed  in  the  reservoir 
of  the  thermometer  is  given  by  the  difference  of  level  of  the 
mercury  in  these  superposed  tubes  B  and  R. 

The  measurement  of  the  pressures  is  made  by  means  of  a 
cathetometer  furnished  with  three  telescopes,  each  of  which  is 
provided  with  a  micrometer  and  level.  The  micrometer  circle  is 


42  HIGH  TEMPERATURES 

divided  into  100  parts;  at  the  distance  from  which  the  manometer 
is  read,  each  division  of  the  circle  corresponds  to  about  0.002  mm. 

The  method  adopted  for  the  measurement  of  pressures  consists 
in  determining  the  position  of  each  mercury  meniscus  in  terms 
of  a  fixed  scale,  hung  near  the  manometer  tubes,  at  the  same 
distance  as  these  latter  from  the  telescopes  of  the  cathetometer. 

One  of  the  principal  difficulties  arising  in  the  measurement  of 
pressures  is  that  of  the  lighting  of  the  menisci.  The  method 
employed  by  Chappuis  consists  in  bringing  up  to  the  surface  of 
the  mercury  an  opaque  point  until  its  image  reflected  by  the 
mercury  appears  in  the  observing  telescope  at  a  very  small  dis- 
tance from  that  of  the  point  itself.  These  two  images  being 
almost  in  contact,  it  is  easy  to  set  the  micrometer  cross-wire 
midway  between  them,  at  the  precise  point  where  would  be  the 
image  of  the  reflecting  surface.  In  order  to  have  a  very  sharp 
image  of  the  point,  it  is  well  to  illuminate  from  behind  by  means 
of  a  beam  of  light  passing  through  a  vertical  slit.  The  point  and 
its  image  then  stand  out  black  on  a  bright  background.  The 
use  of  a  stylus  of  black  glass  is  preferable  to  that  of  a  steel  point 
on  account  of  unchangeableness  and  of  the  greater  sharpness  of 
the  edges. 

The  method  with  stylus  cannot  be  advantageously  employed 
except  in  wide  tubes,  where  the  reflecting  surface  of  the  mercury 
which  aids  in  the  formation  of  the  image  does  not  have  a  sensible 
curvature. 

Dead  Space.  —  This  consists  of  the  space  occupied  by  the  gas : 
(i)  in  that  part  of  the  capillary  tube  which  does  not  undergo  the 
same  variations  of  temperature  as  the  thermometric  bulb;  (2) 
within  the  piece  of  steel  forming  the  plug  which  caps  the  closed 
arm  of  the  manometer;  (3)  in  the  manometer  tube  between  the 
mercury  and  the  horizontal  plane  in  which  ends  the  piece  of 
steel.  The  mercury  is  supposed  to  just  touch  the  stylus  serv- 
ing as  reference  mark. 

The  capacity  of  the  capillary  tube  has  been  determined  by 
mercury  calibration;  it  was  found  equal  to  0.567  c.c.  The  length 
of  the  capillary  tube  being  i  m.,  if  we  deduct  from  this  capacity 


GAS   PYROMETER  43 

that  of  3  cm.  of  the  tube  which  are  exposed  to  the  same  tempera- 
tures as  the  reservoir,  that  is  0.015  c.c.,  this  leaves  0.552  c.c. 

The  capillary  tube  fits  for  a  length  of  27  mm.  into  the  piece  of 
steel  serving  as  plug.  The  total  thickness  of  this  plug  is  28.3 
mm.;  thus  the  portion  of  the  canal  included  between  the  end  of 
the  capillary  tube  and  the  lower  face  of  the  plug  is  1.3  mm.  in 
length.  As  its  diameter  is  1.35  mm.,  the  capacity  of  this  canal 
is  0.0019  c.c. 

The  space  included  between  a  cross  section  of  the  manometer 
tube  passing  through  the  stylus  and  the  plane  surface  of  the  plug 
is  0.3126  c.c. 

To  have  the  total  volume  occupied  by  the  gas  it  is  necessary  to 
add  as  well  to  this  space  the  volume  of  the  depressed  mercury  in 
the  manometric  tube  caused  by  the  curvature  of  the  meniscus. 
The  radius  of  this  tube  being  equal  to  12.235  mm.,  we  find  for 
this  volume  0.205  c.c. 

We  thus  have  as  the  total  of  the  dead  space  the  sum  of  the 
following  volumes : 

C.c. 

Capacity  of  capillary  tube 0.5520 

Volume  of  canal  in  the  plug 19 

Capacity  of  the  manometer  tube  between  the  stylus  and 

the  plane 3126 

Volume  of  depressed  mercury 2050 


Total  dead  space i  .0715 

When  the  mercury  does  not  just  touch  the  stylus,  we  shall  have 
to  add  to  this  value  0.4772  c.c.  per  millimeter  separation  of  the 
stylus  from  the  top  of  the  meniscus. 

The  expansion  of  the  metal  of  the  bulb  was  measured  by  Fizeau's 
method;  this  volume  was  found  to  have  at  different  temperatures 
the  following  values: 

liters. 


—  20 

o 

20 

40 

60 

80 

TOO 


.03846 
.03899 
.03926 
.04007 
.04061 
.04117 
.04173 


44  HIGH  TEMPERATURES 

The  variation  of  the  capacity  of  the  bulb  due  to  changes  of 
pressure  has  also  been  studied;  per  millimeter  of  mercury  it  is 
0.02337  nim.3;  or 

For     o  mm o  mm:3 

'    100          .2.3 

'      200  4-7 

'      300  7.0 

'    400  9-3 

The  zero  is  verified  from  time  to  time  by  bringing  the  bulb  to 
the  temperature  of  melting  ice;  there  is  absolute  constancy  even 
after  heating  to  100°.  The  deviation  is  at  the  most  0.03  mm. 
for  a  pressure  of  995  mm. 

Chappuis  made  a  most  careful  calibration  of  four  mercury  in 
verre  dur  thermometers  in  terms  of  this  standard  gas  thermometer 
in  an  apparatus  such  as  shown  in  Fig.  2,  and  these  mercury 
thermometers,  with  copies  that  have  been  made  and  distributed, 
represent  to-day  the  practical  standards  of  temperature  in  the 
interval  —  35°  to  +  100°  C.,  with  an  accuracy  of  about  0.002°  C. 

After  a  discussion  of  the  formulae  involved,  we  shall  consider 
the  question  of  the  experimental  establishment  of  the  high 
temperature  scale,  a  problem  which  has  occupied  a  great  many 
able  investigators  for  many  years,  and  which  is  by  no  means  as 
yet  conclusively  solved,  there  being,  as  we  shall  see,  embarassing 
outstanding  uncertainties  in  determinations  of  temperatures, 
for  example,  of  0.5°  at  500°  C.  and  some  20°  at  1600°  C.,  due 
wholly  to  experimental  difficulties. 

Formulae  and  Corrections.  —  To  illustrate  the  principles  in- 
volved we  shall  cite  as  examples  some  of  the  earlier  work  with 
porcelain  bulbs.  As  we  shall  see  later,  all  of  the  errors  here 
discussed  have  been  greatly  reduced  in  magnitude  in  the  latest 
work  with  quartz  and  metal  bulbs. 

i.  Thermometer  at  Constant  Volume.  — We  must  now  render 
more  precise  the  formula  of  the  gas  thermometer  given  in  the 
preceding  chapter  by  taking  account  of  the  variations  of  volume 
of  the  bulb,  of  the  surrounding  air  temperature  which  changes 
the  density  of  the  mercury,  and  finally  of  the  volume  of  the 
dead  space. 


GAS   PYROMETER  45 

We  have  three  series  of  observations  to  make  in  order  to 
determine  a  given  temperature: 

PoF0  =  n0RT0, (i) 

(2) 

.    .  (3) 

Putting 


the  first  two  series  serve  to  determine  -• 

It  is  preferable,  except  in  researches  of  very  great  precision,  to 
take  -  from  previously  obtained  results,  and  not  to  make  the 

observations  at  100°,  unless  one  does  so  to  check  his  experimental 
skill. 

Dividing  the  third  equation  by  the  first,  we  have  the  relation 

PV        ffApF        nRT        nT  (} 

r>    T7     ~  ~     E7    A  17  i     J?T>     ~      —    T>    y     '      '        *        '        \4/ 

z  o  '  0          LlQi\V  o          flQ£\.J.  o 


where  H  and  H0  are  the  heights  of  mercury,  A  and  A0  the  den- 
sities of  this  metal. 

For  a  first  approximation  let  us  neglect  the  differences  between 
V  and  FO,  n  and  n0,  A  and  A0.  We  shall  have  then  an  approxi- 
mate value  T'  for  the  temperature  sought: 

7*  -  l    .    —  (^ 

r  '        ' (5) 


a 
for 


Let  us  find  now  the  correction  dT  to  T'  to  obtain  the  exact 
temperature.  In  order  to  find  this,  take  the  logarithmic  dif- 
ferential of  (4) : 

dT     </A  LdF      dn  ff. 

r  =^+F7"^'     (6) 


46  HIGH  TEMPERATURES 

Then  determine  the  values  of  the  different  terms;  let  t\  and  fe  be 
the  absolute  temperatures  of  the  surroundings  when  the  bulb  is 
at  the  temperatures  T'  and  TQ. 

dA  _  A  -  AQ 
i.  > 

AO  AQ 

A  =  A0-[l  -  k(k  -ti)l 
k  =  0.00018(^2  —  4), 

^  =  -  0.00018  (k  -  h). 
AO 

dV      V  -  FQ 
F0  =        Fo 

V  =  F0[l  +  £'(r-  To)], 
&'  (porcelain)  =  0.0000135, 

-—  =  0.0000135  (r  -  r0), 

^0 

by  neglecting  the  variations  of  volume  of  the  bulb  due  to» 
changes  of  pressure. 

_  dn  _  x<i  —  x\ 
n0          n0 

in  calling  #2  and  Xi  the  number  of  molecules  contained  in  the 
dead  space  e  at  the  temperatures  /2  and  t\.  We  have,  in  fact, 
N  being  the  total  mass  contained  in  the  apparatus, 

n  =  N  —  x%, 
n0  =  N  -  xi, 


To  determine  x\  and  x<i\ 

Pot  = 


n0 


GAS   PYROMETER  47 

In  noting  that 

^p  =  r 

Po  ~  To' 
we  have 


Put 

,  .  .  fr  +  fe 

~~ 


__  2 

After  substitution  we  have 

T'  -  To      0    T' 


^=      e   / 

w0  "  "  FO  \ 


These  successive  transformations  are  for  the  purpose  of  mak- 
ing evident  from  the  formula: 

1.  The  ratio  of  the  dead  space  to  the  total  volume:  —  ; 

*  0 

2.  The  temperature  measured:  Tf  —  T0; 

3.  The  variation  of  the  surrounding  temperature  0; 

which  are  the  three  essential  factors  on  which  depends  the 
correction  in  question. 
Formula  (6)  then  becomes: 

p-  =  -  0.00018  fe  -  /i)  +  0.0000135  (r  -  TO) 

T  -  To     e  T  - 


_e_    IT  -  TQ  _0    T  -  Tp\ 

~  v0\   t      r    t   i 


Let  us  take  a  numerical  example  in  order  to  show  the  impor- 
tance of  these  correction  terms  in  the  three  following  cases: 

r  -  TO  =  500°, 
r  -  TO  =  1000°, 
r  -  TO  =  1500°. 


48  HIGH  TEMPERATURES 

In  taking 


t  =  27°  +  273°  =  300°, 

2  e  =  10°, 

we  have 

dT600   =-     1.4°+    5.15°  +  13.1°=    16.85°, 

dT1QOQ  =  -     2.3°  +  17.0°    +  38.2°  =    52.9°, 
dTmo  =  -  30.7°  +  35.7°    +  90.0°  =  122.5°. 

These  figures  show  the  very  great  importance  of  the  dead  space, 
whose  exact  volume  it  is  very  difficult  to  determine.  This 
method  of  computation  of  the  corrections  by  logarithmic  dif- 
ferentials is  only  approximate,  and  is  not  sufficient  for  real 
measurements,  but  it  renders  more  clear  the  general  discussion 
of  the  causes  of  error. 

Let  us  see  what  uncertainty  in  the  temperature  may  result 
from  the  uncertainty  which  there  may  be  in  the  volume  of  the 
dead  space.  In  reality  there  is  a  continuous  passage  from  the 
high  temperature  of  the  pyrometer  to  the  surrounding  tempera- 
ture on  a  length  which  may  vary  from  10  to  30  centimeters, 
according  to  the  thickness  of  the  walls  of  the  furnace.  The 
volumes  of  the  bulb  and  of  the  dead  space  which  should  be  taken 
in  order  that  the  above  formulas  be  exact  should  be  such  that  the 
real  pressure  is  equal  to  the  pressure  that  would  exist  in  supposing 
that  a  complete  and  sudden  change  of  temperature  took  place  at 
a  definite  fictitious  point,  separating  the  heated  part  from  the 
cold  part  of  the  apparatus.  The  probable  position  of  this  point 
is  estimated,  and  if  the  estimation  is  poorly  made,  two  errors  are 
committed,  one  on  the  real  volume  heated  and  the  other  on  the 
dead  space,  errors  equal  and  of  opposite  sign  so  far  as  the  volume 
is  concerned. 

To  calculate  this  error,  as  in  the  case  of  the  corrections,  we 
may  employ  the  method  of  logarithmic  differentials. 


GAS   PYROMETER  49 

Applying  the  same  formula  as  before,  we  find  for  the  relative 

dT 
error  —  : 

dT=_  dV  IT'  -  To  _  6   r  -  TV 
T  ~~     '  V0\       t  ~t"      t 

and  neglecting  the  second  term  of  the  parenthesis,  which  is 
relatively  very  small, 

dT=      dV  IT'  -  TV 
T        ~  V0(       t 

Letting  the  section  of  the  capillary  tube  be  equal  to  i  sq.  mm., 
the  volume  of  the  bulb  100  c.c.,  and  assuming  an  uncertainty  of 
100  mm.  in  the  position  of  the  transition  point,  a  value  often  not 
exaggerated,  we  find  the  following  errors  in  the  temperatures: 

dT6W   =  1.7°, 

jrrt         ~  ^.o 

»*'•*• 1000  —  o'y  ) 

dTmQ  =  8.5°. 

We  thus  see  that  at  1000°  the  error  resulting  from  the  uncer- 
tainty in  the  origin  of  the  dead  space  may  reach  several  degrees 
for  a  bulb  of  100  c.c. 

A  second  cause  of  error  results  from  the  changes  of  mass  fol- 
lowing the  ingoings  and  outgoings  of  gas.  As  before,  we  have 

dT      _  dn 

HQ 

Consider  the  experiments  of  Crafts.  There  enters  per  hour 
at  1350°  in  a  bulb  of  porcelain  of  100  c.c.,  0.002  grm.  of  water 
vapor,  or  0.225  milligram  molecules;  the  initial  volume  inclosed 
at  the  start  is  4.5  milligram  molecules: 

dT      0.225  _ 

which  leads  to  an  error  of 

dTiMo*  =  70°  (about) 
for  an  experiment  lasting  one  hour. 


50  HIGH  TEMPERATURES 

This  computation  demonstrates  clearly  the  enormous  errors 
which  may  result  from  the  penetration  of  an  outside  gas  during 
the  time  of  one  hour,  a  length  of  time  much  less  than  that  of  an 
ordinary  experiment.  It  is  true  that  this  error  decreases  rapidly 
with  rise  of  temperature,  and  it  is  very  probably  zero  at  1000°,  if 
there  is  no  break  in  the  glazing. 

2.  Constant-pressure  Thermometer.  —  We  still  employ  the  same 
formula  (4): 

HAV          nRT 


which  gives  for  a  first  approximation 

H  =«o. 
T0       n 

Calling  /i  and  k  the  surrounding  absolute  temperatures  corre- 
sponding to  TQ  and  TI,  HI  and  14%  the  corresponding  volumes  of 
the  dead  space  and  of  the  reservoir,  we  have,  for  the  determina- 
tion of  n  and  n§,  the  relations: 


n  =  N  —  x2  =  n0  —  (x2  —  Xi), 


As  before,  there  is  a  correction  to  be  applied  to  the  approximate 
temperature  T  thus  obtained  : 

dT  _dH      dA      dV 
Tf    "  #0      Ao       Fo' 

an  expression  the  values  of  whose  terms  are  known. 
Let  us  see  now  the  causes  of  error  and  discuss  their  importance. 
The  error  resulting  from  the  uncertainty  in  the  boundary  of 
the  hot  and  cold  volumes  is 

dT  =  dnv  _  dn  =  dnf    _  T\          dn0/T'  -  T 
T'       nQ        n       nQ\        To)          n0\      TQ 


GAS   PYROMETER  51 

As  before,  let 

dn         i 


n0      1000 
Then  we  find 

dTw   =  1.5°, 

dTiooo  =  5.0°, 
dTiwQ  =  9.3°. 

Thus  the  errors  due  to  this  cause  are  still  greater  than  by  the 
method  of  constant  volume. 

In  order  to  make  exactly  the  correction  for  the  dead  space, 
the  method  of  Regnault's  compensator  may  be  employed,  as 
in  the  work  of  Sainte-Claire-Deville  and  Troost;  this  allows  of 
placing  the  measuring  apparatus  at  a  considerable  distance  from 
the  fire,  which  makes  the  experiments  much  easier. 

Let  us  now  examine  the  error  resulting  from  the  entrance  of 
exterior  gases: 

dT  _  dn  _  dno    T^ 
T       n        n0    TQ 

For  the  experiment  of  Crafts,  the  error  would  be  413°  instead 
of  70°,  the  bulb  being  filled  at  the  start  at  atmospheric  pressure. 

It  is  thus  evident  that,  from  all  points  of  view,  the  method 
of  constant  volume  is  more  precise  than  that  of  constant  pres- 
sure; the  lack  of  impermeability  of  the  coverings  was  the  only 
hindrance  preventing  the  use  of  the  former  in  early  practice. 

3.  V olumenometric  Thermometer. — The  volumenometer  of 
Becquerel  does  not  require  the  invariability  of  the  gaseous  mass 
throughout  the  duration  of  the  experiment.  The  method  con- 
sists in  measuring  the  changes  of  pressure  resulting  from  a  given 
variation  of  the  gaseous  mass  contained  in  the  bulb.  Becquerel 
employed  very  slight  changes  of  mass;  the  changes  of  pressure 
are  then  equally  slight,  which  diminishes  the  precision  of  the 
measurements. 

There  is  no  theoretical  inconvenience  in  reaching  an  absolute 
vacuum,  or,  what  is  practically  more  simple,  using  the  exhaustion 
given  by  a  water  pump,  as  was  done  by  Mallard  and  Le  Chate- 


52  HIGH  TEMPERATURES 

lier;  this  considerably  increases  the  precision.  If  the  exhaustion 
is  complete,  we  have  the  relation 

PV_          _  Potto 
RT'~          RT0' 

UQ  being  the  volume  of  the  reservoir  corresponding  to  the  sur- 
rounding temperature  T0.  If  the  two  volumes  are  filled  under 
atmospheric  pressure,  P  =  PO,  and  then 

r  =  u 

T0      V 

There  are  two  corrections  to  make:  the  first  relative  to  the 
expansion  of  the  envelope,  the  second  to  the  difference  between 
P  and  PQ  when  the  exhaustion  is  produced  by  a  water  pump: 

dT  =  dPdV 

r  ™  P     F  ' 

In  general  dP  is  in  the  neighborhood  of  15  mm.  of  mercury, 
which  gives 

dP 

—  =  0.02. 

Also, 

/1V 

^=  0.0000135  (r  -r0), 
^  =  -  0.02  +  0.0000135  (r  -  r0). 

Calculating  this  correction  for  different  temperatures,  we  have 
dT500   =  -  10.4°, 
dTiQoo  =  -    8.5, 
dTmo  =  —    0.35. 

Let  us  compute  now  the  error  which  comes  from  the  uncer- 
tainty in  the  position  of  the  line  of  separation  of  the  warm  part 
and  the  cold  part  of  the  apparatus;  it  is,  besides,  the  only  remain- 
ing one: 

dT  =  dV_ 

r  ~~   v' 


GAS   PYROMETER  53 

As  before,  assuming  the  higher  limit  to  be 

dT         i 


T      1000  ' 
which  leads  to 

dT500  =  0.77°, 

dTiooo  =  1.27, 

dTiwo  =  2.77. 

From  the  point  of  view  of  the  reduction  of  these  errors,  this 
method  is  preferable  to  the  others,  but  it  appears  to  have  the 
theoretical  disadvantage  of  not  being  reducible  to  the  thermo- 
dynamic  scale. 

This  whole  discussion  of  the  sources  of  error  in  the  measure- 
ment of  temperatures  aims  merely  to  obtain  a  determination 
of  the  temperature  of  the  pyrometer  employed.  But  this  tem- 
perature is  in  itself  not  the  real  object  of  the  measurements;  it 
is  but  an  intermediary  to  arrive  at  a  knowledge  of  the  tempera- 
ture of  certain  other  bodies  supposed  to  be  in  thermal  equilibrium 
with  the  pyrometer.  Now  this  equilibrium  is  extremely  difficult 
to  realize,  and  it  is  more  often  the  case  that  there  is  no  way  of 
being  sure  of  the  exactitude  with  which  it  has  been  obtained 
Here  is  then  a  source  of  error  very  important  in  the  measure- 
ment of  temperatures,  especially  of  high  temperatures,  at  which 
radiation  becomes  an  important  consideration.  Within  an  in- 
closure  whose  temperature  is  not  uniform,  which  'is  true  for 
the  majority  of  furnaces,  there  may  exist  enormous  differences 
of  temperatures  between  neighboring  parts.  One  cannot  too 
strongly  insist  upon  the  presence  of  this  source  of  error,  with 
whose  existence  too  many  investigators  have  not  sufficiently 
occupied  themselves. 

Substance  of  the  Bulb.  —  One  of  the  most  important  points 
to  consider  is  the  choice  of  the  substance  which  constitutes  the 
bulb;  it  is  necessary  to  know  its  expansion  to  account  for  the 
variation  of  its  volume  under  the  action  of  heat;  and  one  must 
be  sure  of  its  impermeability  to  gases  under  pressure. 


54  HIGH  TEMPERATURES 

The  following  substances  have  been  used  up  to  the  present 
time  to  make  these  bulbs:  platinum  and  its  alloys,  iridium,  iron, 
porcelain,  glass,  and  fused  quartz. 

Platinum,  in  spite  of  its  high  price,  was  employed  by  Pouillet 
and  Becquerel;  it  has  the  advantage  over  iron  in  not  being 
oxidizable,  over  porcelain  in  not  being  fragile.  Its  coefficient 
of  expansion  increases  in  a  regular  manner  with  temperature: 

Between  o°  and  100°.     Between  o°  and  1000°. 
Mean  linear  coefficient 0.000007  0.000009 

In  the  course  of  a  noted  controversy  between  H.  Sainte-Claire- 
Deville  and  E.  Becquerel,  the  former  of  those  savants  discovered 
that  platinum  was  very  permeable  to  hydrogen,  a  gas  whose 
presence  is  frequent  in  flames  at  points  where  the  combustion  is 
not  complete.  Unfortunately,  platinum  was  accordingly  com- 
pletely abandoned.  It  is  possible,  in  very  many  cases,  to  be  sure 
of  the  absence  of  hydrogen,  and  the  very  precise  experiments  of 
Randall  showed  that  red-hot  platinum  was  quite  impermeable 
to  all  gases  other  than  hydrogen,  even  with  a  vacuum  inside  the 
apparatus.  With  electric  heating  there  is  no  danger  of  attack 
of  metal  bulbs  by  furnace  gases,  as  was  feared  by  the  early 
observers  using  other  heating  methods. 

Alloying  platinum  with  iridium  or  rhodium  greatly  stiffens  the 
bulb,  and  Chappuis  used  a  platinum-iridium  bulb  of  a  liter 
capacity  in  his  researches  on  the  normal  gas  scale;  and  in  the 
later  investigations  of  Holborn  and  Day  at  the  Reichanstalt  in 
their  comparison  of  thermocouple  indications  with  the  nitrogen 
scale  up  to  1150°  C.,  bulbs  of  this  material  replaced  porcelain  to 
great  advantage.  Alloys  of  10  and  20  per  cent  of  iridium  were 
used,  giving  an  extremely  rigid  bulb  and  with  the  walls  only  0.5 
mm.  thick,  one  which  undergoes  no  appreciable  deformation  after 
being  subjected  to  the  considerable  pressures  required  in  the 
constant-volume  gas  thermometer  at  high  temperatures.  This 
alloy  is  also  impermeable  to  nitrogen,  but  must  be  guarded 
against  reducing  gases  and  silicates  at  high  temperatures. 

Holborn  and  Day  also  determined  the  coefficients  of  expansion 
of  platinum,  as  well  as  other  metals,  alloys,  and  porcelain. 


GAS   PYROMETER 

For  platinum  and  platinum-iridium  they  found: 


55 


Platinum  :  X  -  io9  =  8868 1  +  1.324/2  from  o°  to  1000°; 
80  Pt  •  20  Ir  :  X  •  io9  =  8198  /  +  1.418  /2  from  o°  to  1000°, 

These  determinations  were  made  on  bars  nearly  50  cm.  long 
in  a  most  carefully  constructed  comparator  heated  electrically. 
The  uniformity  of  the  expansion  of  platinum  is  shown  by  the 
fact  that  Benoit's  determination  by  Fizeau's  method  in  the 
interval  o°  to  80°  C.  gave 

X-io9  =8901  t+  i.2i/2. 

So  that  in  this  case  extrapolation  of  over  900°  C.  led  to  no 
serious  error. 


Fig.  4.     Apparatus  for  Linear  Expansion. 

Day  and  Sosman,  at  the  Geophysical  Laboratory  of  the  Car- 
negie Institution,  have  measured,  with  an  improved  apparatus 
(Fig.  4)  similar  to  that  of  Holborn  and  Day,  the  expansion  of 
the  platinum  alloys  containing  io  per  cent  (10.6  by  analysis)  of 


56  HIGH  TEMPERATURES 

indium  and  20  per  cent  of  rhodium,  respectively,  which  they  have 
used  as  gas- thermometer  bulbs': 

90  Pt  •  10  Ir  :  X  .  io9  =  8841  /  +  1.306  /2  from  300°  to  1000°  C. 
So  Pt  •  20  Rh  :  X  •  io9  =  8790 1  +  1.610  /2  from  300  to  1400 

It  is  of  interest  to  note  that,  according  to  these  measurements, 
the  coefficient  of  expansion  of  a  platinum  alloy  of  intermediate 
composition  cannot  be  predicted  by  simple  interpolation;  there- 
fore in  any  new  work  the  actual  coefficient  should  be  determined. 
Holborn  and  Valentiner  find,  by  a  less  rigorous  method,  however, 
that  the  expansion  formula  above  given  for  80  Pt  •  20  Rh  may 
be  used  to  1600°  without  serious  error.  The  rhodium  alloy  of 
platinum  was  substituted  for  that  of  iridium  at  the  Geophysical 
Laboratory  for  the  gas-thermometer  bulb  material  because  the 
evaporation  of  iridium  out  of  the  alloy  was  found  to  be  very 
troublesome  by  contaminating  the  thermocouple  wires,  in  their 
colder  parts  particularly,  and  so  changing  the  E.M.F.  readings. 
The  rhodium  alloy  is  less  objectionable  in  this  respect. 

Iridium.  —  Only  one  series  of  measurements  has  been  made 
with  a  bulb  of  iridium,  that  of  Holborn  and  Valentiner,  and  while 
it  appeared  to  give  results  comparable  with  an  alloy  bulb,  it  is 
probably  better  to  use  the  80  Pt  •  20  Rh  bulb  for  the  reason  above 
given.  Also,  iridium,  besides  evaporating  rapidly  at  high  tem- 
peratures, is  very  brittle.  They  find  for  iridium  to  1600°  the 
expansion  coefficient 

X-io9  =  6697  /+  1.158/2. 

Iron  has  but  one  apparent  advantage,  its  cheapness;  it  is  as 
permeable  to  hydrogen  as  is  platinum;  it  is  not  merely  oxidizable 
in  the  air,  but  is  besides  attacked  by  carbonic  acid  and  water 
vapor.  Thus  the  only  gas  -that  can  be  used  with  iron  is  pure 
nitrogen,  and  even  this  is  questionable.  The  coefficient  of  ex- 
pansion of  iron  is  greater  and  increases  more  rapidly  than  that 
of  platinum: 

Between  o°  and  100°.      Between  o°  and  1000°. 

Mean  linear  coefficient 0.000012  0.000015 

Also,  this  increase  is  not  regular;  there  is  produced  at  850°,  at 


GAS   PYROMETER  57 

the  instant  of  the  allotropic  transformation,  a  sudden  change  of 
length,  a  contraction  of  0.25  per  cent. 

It  is  very  difficult  to  obtain  pure  iron;  very  small  quantities  of 
carbon  modify  somewhat  the  value  of  the  coefficient  of  expansion. 
Besides,  the  change  of  state  of  steel  at  700°,  corresponding  to 
recalescence,  is  accompanied  in  the  heating  by  a  linear  contrac- 
tion, varying  with  the  amount  of  carbon  present,  from  0.05  to 
0.15  per  cent. 

Iron,  therefore,  cannot  be  considered  seriously  for  work  of  any 
precision,  and  as  the  only  excuse  for  working  with  the  gas  ther- 
mometer at  all  is  to  obtain  the  highest  possible  accuracy,  no 
substance  should  be  used  for  a  bulb  which  presents  any  serious 
defects. 

Porcelain  was  adopted  as  a  result  of  the  discussion  between 
H.  Sainte-Claire-Deville  and  Becquerel;  it  was  considered  as 
absolutely  impermeable,  but  without  decisive  tests. 

Even  well-baked  porcelain  consists  of  a  paste  somewhat  porous 
and  permeable;  it  is  only  the  glazing  that  assures  its  imperme- 
ability. But  this  covering  may  sometimes  not  be  whole;  as  it 
softens  above  1000°,  it  is  susceptible  of  cracking  if  left  for  a  con- 
siderable time  with  an  excess  of  pressure  on  the  interior  of  the  ap- 
paratus. According  to  Holborn  and  Wien,  the  glazing  is  broken 
after  reaching  1100°,  when  a  considerable  difference  of  pressure 
is  established  in  the  direction  of  the  lifting  up  of  this  glazing. 

Finally,  like  all  verres,  porcelain  dissolves  gases,  and  in  par- 
ticular water  vapor,  which  passes  through  it  quite  readily.  A 
pyrometer  left  a  long  time  in  the  flame  at  about  1200°  becomes 
filled  with  water  vapor,  which  can  be  seen  to  condense  in  the 
manometer  after  a  few  weeks. 

The  experiments  of  Crafts  have  shown  that  the  rapidity  of 
the  passage  of  water  vapor  through  porcelain,  in  a  pyrometer 
of  from  60  to  70  c.c.  capacity  at  the  temperature  of  1350°,  was 
0.002  grm.  of  water  vapor  per  hour. 

It  is  thus  not  safe  to  employ  porcelain  at  temperatures  higher 
than  1000°,  at  least  not  in  the  thermometric  processes  which 
suppose  the  invariability  of  the  gaseous  mass. 


HIGH  TEMPERATURES 


The  expansion  of  porcelain  has  been  the  object  of  a  great 
number  of  measurements  which,  for  porcelains  of  very  different 
make,  give  values  near  to  one  another;  the  mean  linear  coefficient 
between  o°  and  1000°  varies  between  0.0000045  an<^  0.000005 
for  hard  porcelain  —  that  is  to  say,  baked  for  a  long  time  at  a 
temperature  in  the  neighborhood  of  1400°. 

Here  are  the  results  of  experiments  made  by  Le  Chatelier  and 
by  Coupeaux;  the  experiments  were  made  with  porcelain  rods 
i oo  mm.  in  length,  and  the  numbers  express  the  elongation  of 
these  rods  in  millimeters : 


Porcelain. 

Temperatures. 

0° 

200° 

400° 

600° 

800° 

1000° 

Ba.yeux 

0.075 
.078 
.076 
.090 

0.166 
.170 
.168 
.188 

0.266 
.270 
.268 
.290 

0.367 

.378 
.360 

•  390 

0.466 
.470 
.465 
.490 

Sevres  dure  (cuite  a  1400°) 

Limoges           ... 

Sevres  nouvelle  (cuite  a  140x3°)  .... 

These  numbers  should  be  multiplied  by  three  to  give  the  cubical 
expansion. 

Porcelain  has  still  another  inconvenience;  the  glazing  is  usually 
put  on  the  outside  only  of  vessels,  so  that  the  porosity  of  the  paste 
gives  an  uncertainty  due  to  the  unequal  absorption  of  gases  at 
increasing  temperatures. 

According  to  Barus,  it  is  impossible  to  fill  with  dry  air  a  py- 
rometer, not  glazed  inside,  at  ordinary  temperatures.  The  water 
is  not  driven  out  by  pumping  out  several  times  and  letting  in  dry 
air.  An  apparatus  filled  in  this  way  will  indicate  between  melt- 
ing ice  and  boiling  water  from  150°  to  200°.  Nor  is  filling  the 
apparatus  at  100°  satisfactory:  it  will  indicate  115°  for  this  same 
interval  of  100°.  Barus  thinks  that  at  400°,  by  repeating  the 
operation  several  times,  one  can  consider  the  apparatus  as  filled 
with  dry  air. 

The  use  of  porcelain  bulbs  in  several  recent  pyrometric  re- 
searches of  great  importance  has  been  a  cause  of  outstanding 
differences  in  the  determination  of  fixed  points  in  pyrometry  as 


GAS   PYROMETER  59 

the  sulphur  boiling  point,  differences  that  are  due  mainly  to  the 
uncertainties  in  the  expansion  coefficient  of  the  particular  samples 
of  porcelain  used. 

The  work  of  Chappuis,  Tutton,  Bedford,  and  of  Holborn,  Day, 
and  Griineisen  has  shown  the  expansion  of  porcelain  to  be  anoma- 
lous, and  that  therefore  extrapolation  for  the  coefficient  cannot 
be  made  safely  even  over  a  hundred  degrees  for  the  most  exact 
work.  There  is  always  a  deformation  of  the  bulbs  of  uncertain 
and  irregular  amounts  in  a  constant- volume  thermometer  suffi- 
cient to  render  results  doubtful  at  temperatures  as  low  as  500°  C., 
and  Holborn  and  Day  were  unable  with  porcelain  bulbs  to  get 
any  considerable  precision  at  1000°  C.,  and  finally  discarded  them 
entirely. 

They  found  for  the  expansion  of  Berlin  porcelain 

\  .  io9=  £2954^  -f-  1.125  P I  from  o°  to  1000°, 

but  this  value  is  too  high  for  temperatures  below  250°  as  indicated 
by  Chappuis;  and  Holborn  and  Griineisen  have  shown  that  at 
about  700°  C.  a  considerable  change  in  the  coefficient  takes  place, 
the  expansion  becoming  more  rapid  at  higher  temperatures. 

It  would  probably  not  be  worth  while  to  make  further  pyro- 
metric  studies  with  porcelain  bulbs,  when  possible  to  avoid  their 
use. 

Glass  cannot  be  used  above  550°  C.,  but  to  500°  C,  it  may 
replace  porcelain  to  advantage  if  Jena  borosilicate  59™  is  used, 
as  the  deformation  after  heating  is  somewhat  less  and  more 
uniform.  The  coefficient  of  expansion  of  this  glass  as  measured 
in  the  form  of  capillary  tubes  by  Holborn  and  Griineisen  is 
X-io9=  5833^  +  0.882/2. 

This  glass  was  used  by  Holborn  and  Henning  in  1911  to  450° 
with  the  gases  nitrogen,  hydrogen,  and  helium,  to  the  last  of 
which  it  is  slightly  permeable. 

Jena  i6IU  glass  was  used  by  Eumorfopoulos  to  445°  C.  Its 
coefficient  of  expansion  in  terms  of  the  value  of  Callendar  and 
Moss  for  the  absolute  expansion  of  mercury  to  300°  is 

1-310-  ioo)Jio-8, 


60  HIGH  TEMPERATURES 

obtained  by  using  the  thermometer  bulb  as  a  mercury- weight 
thermometer  at  o°,  100°,  and  184°.  This  glass  has  a  troublesome 
zero  lag. 

Several  investigations,  using  both  constant-pressure  and  con- 
stant-volume thermometers,  have  been  carried  out  with  glass 
bulbs  to  500°  C. 

Quartz  glass,  in  the  amorphous  or  fused  form,  can  now  be  ob- 
tained in  vessels  of  several  hundred  cubic  centimeters  capacity; 
thanks  to  many  attempts  culminating  "Successfully  in  the  effects 
of  Heraeus,  and  Siebert  and  Kiihn.  The  chemical  and  physical 
properties  in  view  of  its  pyrometric  use  have  been  studied  by 
many  investigators,  Shenstone  being  a  pioneer  in  advocating  its 
use  in  thermometer  bulbs.  Vitrified  quartz  vessels  seem  to  resist 
deformation  to  fairly  high  temperatures,  the  upper  limit  when 
the  interior  is  a  vacuum  being  not  far  from  1300°  C.;  the  sub- 
stance is  appreciably  plastic  at  1500°. 

Fused  quartz  or  silica  glass  is  attacked  by  alkalies,  and  the 
slightest  trace  of  such,  as  from  handling,  may  do  damage  when 
the  heating  is  carried  very  high.  Weak  acids  and  neutral  salts 
are  without  effect  as  shown  by  Mylius,  but  at  high  temperatures 
all  oxides  attack  it.  Heated  with  a  porcelain  tube,  or  even 
alone,  to  temperatures  above  1100°  C.,  the  quart  tends  to  lose 
its  transparency,  cracking  and  changing  over  into  a  crystalline 
structure  which  readily  crumbles  to  the  touch  on  cooling.  Water 
vapor  even  in  traces  hastens  this  process.  Moissan  has  shown 
that  it  is  slightly  soluble  in  a  lead  bath  above  1100°  C.,  and  very 
much  more  so  in  zinc.  Villard  showed  that  it  is  permeable  to 
hydrogen  but  less  so  than  platinum,  nor  does  it  seem  to  occlude 
other  gases.  It  is  also  quite  permeable  to  helium.  Travers 
and  Jaquerod  find  also  that  silica  is  reduced  by  hydrogen  at  high 
temperatures. 

Its  great  advantage  in  gas  thermometry  is  its  lack  of  deforma- 
tion and  its  extremely  small  coefficient  of  expansion,  about  -fj 
that  of  platinum,  or,  more  exactly,  as  determined  by  Holborn 
and  Henning  with  a  comparator, 

X  .  io9  =  540  /  from  o°  to  1000°. 


GAS   PYROMETER  61 

Scheel,  using  a  Fizeau  apparatus,  finds 

X  •  io9  =  322  /  -f-  1.47 12  between  o°  and  100°, 

where  the  curvature  is  of  the  same  order  as  for  metals.  There 
is  some  question,  however,  as  to  the  constancy  of  this  coefficient. 

In  work  at  1000°  C.  the  expansion  correction  is  reduced  from 
over  20°  with  porcelain  or  platinum  to  about  i°,  and  its  uncer- 
tainties become,  therefore,  negligible,  permitting  a  great  increase 
in  accuracy.  Silica  glass  has  been  used  successfully  by  Jaquerod 
and  Perrot  as  a  thermometer  bulb  to  the  melting  point  of  gold 
with  several  gases.  It  cannot  be  used  continuously  as  an  en- 
velope, however,  at  temperatures  much  over  1100°  C.,  for  the 
reasons  stated  above. 

Early  Experimenters.  —  We  shall  study  now  the  experiments 
made  by  various  investigators,  and  we  shall  see  in  what  degree 
the  conditions  of  precision  indicated  in  the  course  of  this  account 
have  been  realized. 

Pouillet.  —  Pouillet  was  the  first  to  make  use  of  the  air  ther- 
mometer for  the  measurement  of  high  temperatures;  he  obtained 
very  good  values  for  the  epoch  at  which  he  worked. 

His  pyrometer  was  made  of  a  platinum  bulb,  of  ovoid  form,  of 
60  c.c.  capacity,  to  which  was  gold-soldered  a  platinum  capillary 


Fig.  5.     Pouillet's  Thermometer. 

tube  of  25  cm.  in  length;  continuous  with  this  tube  was  another  of 
silver  of  the  same  length  fastened  to  the  manometer.  The  joining 
of  the  platinum  and  silver  tubes  was  made  by  means  of  a  metal 
collar  (Fig.  5).  The  dead  space  had  thus  a  volume  of  2  c.c. 

The  manometer  was  made  up  of  three  glass  tubes  embedded  at 
their  lower  ends  in  a  metallic  piece;  the  first  tube  serving  as  a 
measurer  was  graduated  in  cubic  centimeters,  the  second  con- 


62 


HIGH   TEMPERATURES 


stituted  the  manometer  properly  speaking,  and  the  third  served 
to  fill  the  apparatus. 

A  cock  conveniently  placed  permitted  variation  of  the  quantity 
of  mercury  contained  in  the  apparatus  (Fig.  6).  The  principle 
of  this  apparatus  is  the  same  as  that  of  the  more  recent  Regnault 
manometer;  this  latter  differs  from  the  manometer  of  Pouillet 
only  in  the  suppression  of  the  third  tube,  which  is  replaced  by  a 
bottle  joined  to  the  emptying  cock  by  a  rubber  tube. 

Pouillet's  determinations  of  fusing  points 
follow: 

Gold n8o°(too  high  by  115°) 

Silver 1000  (too  high  by  40°) 

Antimony 432  (too  low  by  200°) 

Zinc 423  (good) 

Ed.  Becquerel.  —  This  savant  took  up  and 
continued  the  work  of  Pouillet  with  the  same 
apparatus.  But  at  the  close  of  a  discussion 
with  H.  Sainte-Claire-Deville  on  the  question 
of  the  permeability  of  platinum,  he  made  use 
successively  of  pyrometers  of  iron  and  of  por- 
celain. The  results  obtained  with  platinum 
seem,  however,  to  be  far  the  best. 

Pyr.  of  Pt.      Pyr.  of  Porcelain 

Boiling  point  of  zinc 930°  (good)  890° 

Fusing  point  of  silver. . .  .  960    (good)  916 

Fusing  point  of  gold 1092  1037 


Fig.  6.     Pouillet's 
Manometer. 


The  figures  for  gold  differ  among  them- 
selves by  about  25°. 

Experiments  of  H.  Sainte-Claire-Deville  and  T roost.  —  They, 
after  their  discussion  with  Becquerel,  made  numerous  experi- 
ments with  the  porcelain  air  thermometer;  they  obtained  very 
discordant  results,  which  they  did  not  publish  at  the  time. 

They  placed  the  most  confidence  in  the  determinations  made 
by  the  aid  of  the  vapor  of  iodine  (we  shall  speak  of  this  later); 
but  when  the  inaccuracy  of  this  method  was  pointed  out,  they 
made  known  the  results  that  they  had  obtained  for  the  boiling 
point  of  zinc. 


GAS  PYROMETER  63 

They  employed  a  crucible  of  plumbago  having  a  capacity  of  15 
grm.  of  zinc;  the  metal  was  added  anew  as  fast  as  it  evaporated. 

The  crucible  was  placed  in  a  furnace  filled  with  coal.  Around 
the  pyrometer  was  arranged  a  covering  of  fire  clay;  but  this 
arrangement  was  quite  insufficient  to  eliminate  errors  due  to 
radiation.  The  same  measurements  were  repeated  with  different 
gases.  The  figures  obtained  for  the  boiling  point  of  zinc  range 
from  pi6°  to  1079°  and  seem  to  be  a  function  of  the  nature  of  the 
gas,  which  is  inexplicable. 

Violle.  —  Guided  by  H.  Sainte-Claire-Deville,  whom  his  suc- 
cessive failures  had  instructed  in  the  difficulties  of  the  problem, 
Violle  made  a  series  of  measurements  which  were  among  the  best 
for  a  long  time.  He  made  use  of  a  porcelain  thermometer,  and 
he  worked  simultaneously  at  constant  pressure  and  constant 
volume.  The  agreement  of  the  two  numbers  shows  if  the  mass 
has  remained  constant;  this  is  the  equivalent  of  the  volumetric 
method  of  Becquerel. 

The  most  serious  objection  that  can  be  made  to  these  observa- 
tions concerns  the  uncertainty  of  the  equality  of  temperatures 
of  the  pyrometer  and  of  the  substance  studied  placed  beside  the 
former;  from  this  point  of  view,  however,  these  experiments, 
made  in  the  Perrot  furnace,  were  much  more  satisfactory  than 
those  made  in  coal  furnaces  previously  employed. 

i.  A  first  series  of  determinations  was  of  the  specific  heat  of 
platinum.  A  platinum  mass  of  423  grm.  was  put  into  a  Perrot 
muffle  alongside  the  pyrometer,  and  when  the  mass  was  in  a  state 
of  temperature  equilibrium  it  was  immersed,  either  directly  in 
water  or  in  a  platinum  eprouvette  placed,  opening  upward,  in 
the  midst  of  the  calorimeter  water.  In  the  first  case  the  experi- 
ment was  made  in  a  few  seconds;  in  the  second  it  lasted  fifteen 
minutes,  and  the  correction  was  as  high  as  0.3°  per  10°;  the 
results,  however,  were  concordant.  At  787°  two  experiments 
gave  0.0364  and  0.0366;  mean,  0.0365. 

At  1000°  twelve  experiments  were  made  employing  the  method 
of  immersion;  the  numbers  found  vary  from  0.0375  to  0.0379; 
mean,  0.0377. 


HIGH  TEMPERATURES 


Near  1200°  the  measurements  were  made  at  constant  pressure 
and  at  constant  volume. 


Temperature 
at  Constant 
Volume. 

Temperature 
at  Constant 
Pressure. 

Mean. 

Specific  Heat 
of  Platinum. 

II7I° 
1169 
"95 

Il6S° 
1166 
1192 

1168° 
1168 
"93 

0.0388 
.0388 
.0389 

The  mean  specific  heat  from  his  observations  may  be  repre- 
sented by  the  formula 


C' 


Q      0.0317 


0.000006  •  /, 


and  true  specific  heat  by 


at 


=  0-03I7  +  0.000012  •/. 


Violle  used  these  determinations  to  fix,  by  extrapolation,  the 
fusing  point  of  platinum,  which  he  found  equal  to  1779°.  He 
measured  for  that  the  quantity  of  heat  given  out  by  i  grm.  of 
solid  platinum  from  its  fusing  point  to  o°.  For  this  purpose  a 
certain  quantity  of  platinum  is  melted,  into  which  is  plunged  a 
spiral  wire  of  the  same  metal,  and,  at  the  instant  that  the  surface 
of  the  bath  solidifies,  by  aid  of  this  wire  a  cake  of  solid  platinum 
is  lifted  out  and  immersed  in  the  water  calorimeter. 

The  latent  heat  of  fusion  of  platinum  is  equal  to  74.73  c.  ±  1.5; 
this  number  results  from  five  determinations. 

2.  A  second  series  of  experiments  was  on  the  specific  heat  of 
palladium;  the  determinations  were  made,  in  part  by  comparison 
with  platinum,  in  part  by  the  air  thermometer.  The  results 
obtained  by  the  two  methods  are  concordant. 

The  mean  specific  heat  is  given  by  the  formula 

CQ  =  0.0582  +  o.ooooio  •  /. 
The  true  specific  heat  is  equal  to 

•p  =  0.0582  +  0.000020  •  t. 
The  fusing  point  of  palladium  was  found  equal  to  1500°.    The 


GAS   PYROMETER  65. 

latent  heat  of  fusion  of  palladium,  measured  by  the  same  experi- 
ments, was  found  to  be  36.3  calories. 

.  3.  In  another  series  of  experiments  Violle  determined  the 
boiling  point  of  zinc.  He  employed  an  apparatus  of  enameled 
casting,  heated  in  a  triple  envelope  of  metallic  vapor;  the  top 
was  covered  with  clay  and  cow-hair  to  prevent  superheating  of 
the  coverings.  The  measurements  were  made  with  pressure  and 
volume  simultaneously  variable.  He  found  about  930°. 

4.  A  last  series  is  relative  to  the  fusing  points  of  metals, 
which  were  determined  by  comparison  with  the  total  heat  of 
platinum: 

Silver 954°  (too  low  by    7°) 

Gold 1045    (too  low  by  18°) 

Copper  (probably  saturated  with  Cu2O) .  .   1050    (too  low  by  13°) 

Mallard  and  Le  Chatelier.  —  In  their  investigations  on  the  tem- 
peratures of  ignition  of  gaseous  mixtures,  Mallard  and  Le  Cha- 
telier made  use  of  a  porcelain  pyrometer,  which  was  exhausted; 
then  air  was  let  in  and  the  gaseous  volume  thus  absorbed  was 
measured.  It  is  possible  to  reach  1200°  without  noticing  any 
breaking  down  of  the  porcelain;  but  this  giving  way  is  complete 
at  1300°  under  the  action  of  the  vacuum. 

This  method  was  used  in  the  following  way  to  measure  the 
temperatures  of  ignition  of  gaseous  mixtures:  The  air  was  ex- 
hausted from  the  apparatus,  and  the  temperature  was  meas- 
ured by  the  air  volume  which  filled  it;  the  air  was  again  exhausted 
and  the  apparatus  was  filled  with  the  gaseous  mixture.  Whether 
or  not  there  was  ignition  was  known  by  the  comparison  of  the 
volume  of  the  mixture  with  that  of  the  air  introduced  under  the 
same  conditions  of  temperature,  at  least  in  the  cases  of  mixtures 
burning  with  contraction. 

The  pyrometer  used  had  a  capacity  of  62  c.c.,  after  deduction 
of  the  dead  space  (i  c.c.) ;  the  following  table  gives  the  volumes 
of  air  corresponding  to  different  temperatures: 

400° 26 . 7  c.c. 

600  20.6 

800  16.7 

1000  14.1 

1200  .  .' 12.2 


66 


HIGH  TEMPERATURES 


In  admitting  that  the  measurements  of  volume  be  made  to 
o.i  c.c.,  one  should  have  a  precision  of  only  10°  in  1000°  on 
account  of  the  insufficient  volume  of  the  thermometric  reservoir. 

Barus.  —  This  American  physicist  devised  a  rotating  appara- 
tus, remarkable  for  its  uniformity  of  temperature/but  he  applied 
it  directly  only  to  the  standardization  of  thermoelectric  couples. 
He  worked  at  constant  pressure.  By  means  of  couples  graduated 
in  this  way,  he  determined  the  boiling  points  of  zinc  (926°  to  931°) 
and  of  cadmium  (773°  to  784°);  the  boiling  point  of  bismuth  was 


Fig.  7.    Apparatus  of  Barus. 

found  equal  to  1200°  under  a  reduced  pressure  of  150  mm.,  which 
would  give  under  atmospheric  pressure  by  extrapolation  150x5°. 

Fig.  7  represents  the  longitudinal  section  of  Barus'  apparatus. 
It  is  composed  essentially  of  a  porcelain  pyrometer  containing 
an  interior  tube  in  which  is  placed  the  couple.  The  pyrometer 
fixed  at  a  point  of  its  stem  is  held  stationary.  It  is  surrounded 
by  a  muffle  of  casting  whose  general  shape  is  that  of  revolution 
about  the  axis  of  the  pyrometer;  this  muffle  is  composed  of  two 
similar  halves  held  by  means  of  iron  collars,  and  can  be  given  a 
motion  of  rotation  about  its  axis  of  figure,  in  such  a  manner  as 
to  assure  uniformity  of  heating.  It  is  heated  by  gas  burners 
placed  below.  An  outer  covering  of  fire  clay  keeps  in  the  heat 
about  the  iron  muffle. 


GAS   PYROMETER  67 

Holborn  and  Wien.  —  Holborn  and  Wien  made  a  very  com- 
plete standardization  of  the  thermoelectric  couple  Pt,  90  Pt- 
10  Rh  proposed  by  Le  Chatelier.  They  made  use  of  a  por- 
celain reservoir  of  about  100  c.c.  capacity,  terminating  at  its 
two  ends  in  capillary  porcelain  tubes.  The  thermoelectric  junc- 
tion is  placed  inside  the  bulb,  and  each  of  its  wires  is  led  out  by 
one  of  the  lateral  tubes ;  this  arrangement  allows  of  determining 
at  various  points  the  real  temperature  of  the  dead  space  whose 
volume  is  1.5  c.c. 

They  worked  at  constant  volume,  with  a  very  low  initial  pres- 
sure so  as  always  to  have  depression;  they  were  able  to  reach 
1430°.  Above  1200°  they  could  make  but  a  single  observation 
with  one  pyrometer;  below  this,  about  ten  observations. 

They  determined  very  approximately  the  coefficient  of  ex- 
pansion of  their  porcelain,  a  product  of  the  Berlin  works,  and 
found  it  equal  to  0.0000045,  the  identical  number  given  by  Le 
Chatelier  for  the  Bayeux  porcelain. 

They  made  use  of  this  pyrometer,  employing  as  intermediary 
a  couple,  to  fix  the  fusing  points  of  certain  metals : 

Silver 970° 

Gold 1072 

Palladium 1580 

Platinum 1780 

These  figures,  at  the  time  they  were  obtained,  were  counted 
among  those  which  seemed  to  merit  the  most  confidence;  how- 
ever, it  is  necessary  to  note  that  the  volume  of  the  bulb  was  too 
small  to  assure  a  very  great  accuracy,  and  its  expansion  coefficient 
not  well  known. 

We  shall  return  to  these  experiments  when  treating  of  electric 
pyrometers. 

Recent  Experimental  Investigations.  —  Modern  gas  thermom- 
etry  of  precision  may  be  said  to  begin  with  the  introduction  of 
electric  furnaces,  and  the  discarding  of  porcelain  bulbs,  both  of 
which  were  effected  by  Holborn  and  Day.  The  constant-volume 
thermometer  is  the  one  almost  universally  used  in  the  more  recent 
gas  thermometer  researches  at  temperatures  above  500°  C.,  and 


68  HIGH  TEMPERATURES 

the  inclosed  gas  is  usually  nitrogen.  There  have  been  further 
experiments  at  the  Reichsanstalt,  where  the  work  to  1100°  C. 
was  first  repeated  by  Holborn  and  Day  and  then  carried  to 
1600°  C.  by-Holborn  and  Valentiner.  At  the  Geophysical  Labo- 
ratory in  Washington,  also,  Day,  Clement,  and  Sosman  have 
determined  a  series  of  fixed  points  from  zinc  to  palladium,  using 
greatly  improved  methods  for  the  exact  determination  of  the 
higher  temperatures.  Jaquerod  and  Perrot  have  used  several 
gases  in  quartz  glass  to  the  temperature  of  fusion  of  gold;  and  the 
hydrogen  thermometer  has  been  used  by  Jaquerod  and  Wassmer 
for  the  determination  of  the  boiling  points  of  naphthaline  and 
benzophenone.  The  scale  of  the  platinum-resistance  thermom- 
eter has  been  compared  with  that  of  nitrogen  to  500°  C.,  at  con- 
stant pressure  by  Callendar,  and  at  constant  volume  by  Chappuis 
and  Harker,  and  by  Holborn  and  Henning;  and  from  these 
series  of  measurements  the  boiling  point  of  sulphur  has  been 
determined  by  these  observers  and  also  by  Eumorfopoulos,  using 
Callendar's  form  of  the  constant-pressure  thermometer. 

We  shall  discuss  in  some  detail  most  of  these  recent  researches, 
in  part  here,  and  in  part  in  the  chapter  on  standardization. 

Holborn  and  Day.  —  Their  preliminary  work  was  done  with 
porcelain  bulbs  at  temperatures  above  500°  C.,  using  nitrogen  and 
hydrogen  and  with  a  bulb  of  Jena  borosilicate  glass  No.  59™  filled 
with  hydrogen,  for  temperatures  below  500°.  Porcelain  bulbs 
glazed  outside  and  also  inglazed  bulbs  were  used.  Errors  due  to 
changes  in  the  bulbs  were  detected  by  taking  "zero"  readings 
and  also  by  the  simultaneous  use  of  thermocouples.  Salt  baths 
were  used  up  to  700°  at  first,  but  later  electric  heating  in  air 
was  employed  in  all  the  high-temperature  work. 

The  hard  glass  bulbs  of  about  167  cm.  capacity  showed  less 
changes,  after  annealing,  than  the  irregularities  in  the  thermo- 
couple measurements,  due  to  the  lack  of  sensitiveness  of  the 
latter  at  low  temperatures;  and  these  glass  bulbs  were  found 
preferable  to  those  of  porcelain  up  to  500°  C.  The  precision 
attainable  with  thermocouple  control  was  about  0.6°  C. 

Porcelain  bulbs  of  100  c.c.  capacity,  glazed  inside  and  out,  filled 


GAS   PYROMETER  69 

with  hydrogen,  and  heated  to  only  700°,  gave  very  discordant 
results  due  apparently  to  chemical  action  between  the  hydrogen 
and  the  walls  of  the  bulb  and  to  water  vapor  generated.  Used 
with  nitrogen  and  heated  electrically  to  about  1100°  C.,  the  mean 
difference  between  the  observed  and  calculated  values  was  ±1.5° 
C.  Far  less  satisfactory  results  were  obtained  with  porcelain 
glazed  only  on  the  outside. 

A  first  series  of  experiments  with  a  metal  bulb  were  made  with 
a  20  per  cent  iridium  alloy  of  platinum,  the  bulbs  being  cylindri- 
cal, of  208  c.c.  volume  and  0.5  mm.  wall,  and  the  dead  space  was 
considerably  reduced  over  that  of  the  porcelain  bulbs.  The 
electric  heating  oven  was  also  improved  by  winding  it  logarith- 
mically so  that  at  1150°  the  temperature  distribution  was  con- 
stant to  3°  over  that  portion  of  the  oven  containing  the  bulb. 
This  was  still  further  equalized  by  the  presence  of  the  metallic 
bulb ;  also  at  very  high  temperatures  the  tendency  to  equilibrium 
through  radiation  balances  more  nearly  the  losses  by  end  con- 
duction. Temperature  control  to  0.1°  C.  at  1000°  C.  may  be 
realized  electrically  with  care.  A  precision  of  better  than  i°  C. 
was  then  obtained,  and  the  conclusion  seemed  warranted  that 
the  metallic  bulbs  in  an  electrically  heated  furnace,  where  no 
gases  or  other  materials  acting  upon  platinum  were  in  contact 
with  it,  were  superior  to  any  form  of  porcelain  bulb. 

Their  later  work  consisted  in  a  determination  of  fixed  points, 
using  the  thermocouple  as  intermediary,  after  having  found  the 
coefficient  of  expansion  of  the  material  of  their  bulb  and  shown 
that  the  bulb  underwent  no  deformation  after  heating.  The 
correction  for  expansion  amounts  to  30°  at  1000°  and  40°  at  1150°. 
The  expansion  was  determined  for  a  50  cm.  bar  in  a  comparator 
which  could  be  heated  electrically  to  1000°  C. 

Although  no  change  in  volume  of  the  thin-walled  bulb  could 
be  detected  on  cooling,  a  temporary  yielding  of  the  glowing  walls 
under  the  comparatively  high  pressure  might  have  taken  place, 
so  a  bulb  having  walls  i  mm.  thick  was  substituted,  the  compo- 
sition being  90  Pt-io  Ir.  This  bulb  was  as  satisfactory  as  the 
first. 


70  HIGH  TEMPERATURES 

The  results  obtained  by  Holborn  and  Day  for  the  fixed  points, 
as  well  as  their  work  with  thermoelements,  will  be  discussed  later. 

Jaquerod  and  Perrot.  —  Using  a  quartz  bulb  filled  at  constant 
volume  successively  with  nitrogen,  air,  oxygen,  carbon  monoxide, 
and  carbonic  acid,  and  employing  an  electric  resistance  furnace, 
results  agreeing  to  0.3°  were  obtained  for  the  fusing  point  of  gold 
with  the  first  four  gases,  using  a  common  coefficient  of  expansion 
based  on  Chappuis'  limiting  value  and  using  varying  initial  pres- 
sures. The  use  of  quartz  reduces  the  correction  for  the  expansion 
of  the  bulb  to  2°  at  1000°. 

This  work  shows  that  in  the  range  o°  to  1100°  C.  the  coeffi- 
cients of  expansion  of  these  gases  are  practically  identical  (see 
page  26). 

Callendar 's  Constant-pressure  Thermometer.  —  For  the  calibra- 
tion of  the  platinum-resistance  thermometer  Callendar  has  stud- 
ied an  arrangement  of  the  constant-pressure  gas  thermometer 
in  which  the  dead  space  is  reduced  to  a  minimum  by  an  ingen- 
ious device  which  consists  in  interposing  in  the  capillary  tube 
a  column  of  sulphuric  acid  which  is  always  brought  to  the  same 
position  (Fig.  8).  It  is  then  permissible  to  leave  vacant  spaces 
in  the  manometer  of  any  volume,  and  this  simplifies  the  measure- 
ments. 

The  bulb  is  of  glass,  and  its  capacity  is  77.01  c.c.  The  capillary 
tube  has  a  diameter  of  0.3  mm.  It  is  attached  to  a  small  U  tube 
of  2  mm.  diameter  which  contains  the  sulphuric  acid.  The  total 
value  of  the  waste  space  is  thus  reduced  to  0.84  c.c. 

The  sulphuric  acid  before  each  measurement  is  brought  up  to 
a  reference  mark.  The  density  of  this  liquid  being  one-seventh 
that  of  mercury,  the  errors  made  in  determining  its  level  should 
be  divided  by  seven  to  express  them  in  heights  of  mercury.  The 
use  of  this  column  of  sulphuric  acid  has  the  inconvenience  to 
oblige  the  experimenter  to  watch  constantly  the  apparatus  during 
the  whole  time  of  heating  and  cooling  in  order  to  maintain  the 
pressure  equilibrium  in  the  two  parts  of  this  column;  otherwise 
the  liquid  would  be  driven  into  the  manometer  or  absorbed  into 
the  bulb. 


GAS   PYROMETER  71 

The  manometer  is  one  open  to  the  air  and  is  read  conjointly 
with  the  height  of  the  barometer. 

The  coefficient  of  expansion  of  the  hard  glass  used  in  the  con- 
struction of  the  thermometer  was  measured  for  a  tube  of  same 
make  by  means  of  two  microscopes  carried  upon  a  micrometer 
screw  but  sighted  on  the  cold  ends  of  the  tube.  A  cold  com- 
parison tube  could  be  placed  under  the  microscopes  to  verify  the 
invariability  of  their  distance  apart. 


Thermometer 


Manometer 


Fig.  8.     Calendar's  Differential  Manometer. 


MEAN  COEFFICIENT  OF  EXPANSION. 


102 

222 
330 
48l 


0.00000685 
706 
740 
769 

810 


After  heating  to  400°  there  were  permanent  changes  amounting 
to  from  0.02  to  0.05  per  100. 

If  the  zero  is  taken  at  intervals  of  time  of  varying  length, 


72  HIGH  TEMPERATURES 

permanent  displacements  are  noted  in  the  bulb.     The  following 
table  gives  some  examples: 


Date. 

Oxygen 
Thermometer. 

Nitrogen 
Thermometer. 

Remarks. 

[an.  21,  1886  

mm. 
693.1 

mm. 

(  Filled  at  300°;  measurement 
(      taken  4  days  later. 

an.  22,  1886  
an.  23,  1886  
an.  25,  1886  
an.  25,  1886  

692.9 
692.9 
692  .0 
692.0 

695.1 
694.9 
693-8 
694.1 

After  heating  to  100°. 
After  heating  to  100°. 

This  change  of  zero  has  been  attributed  to  a  partial  absorption 
of  the  air  by  the  glass.  Glass,  an  amorphous  body  resembling 
liquids  somewhat,  may  dissolve  gases,  especially  at  high  tempera- 
tures, although  this  is  not  borne  out  by  Holborn  and  Day's  work 
on  nitrogen. 

For  temperatures  higher  than  300°  this  source  of  error  becomes 
very  serious,  especially  if  the  gas  is  hydrogen.  This  gas  dis- 
appears progressively  by  solution  in  the  glass  or  by  oxidation, 
replacing  elements  of  the  glass.  It  is  necessary  to  revert  to 
nitrogen.  This  fact  was  observed  by  Chappuis  and  Marker  in 
the  course  of  a  study  of  the  platinum-resistance  pyrometer  when 
the  temperatures  measured  reached  as  high  as  600°. 

One  of  the  more  recent  forms  of  this  thermometer  in  which 
there  is  complete  compensation  of  the  dead  space  is  shown  in 
Fig.  9,  where  A  is  the  thermometer  bulb  connected  by  a  capillary 
a  to  an  overflow  bulb,  or,  as  here  shown,  to  a  burette  B.  The 
compensating  capillary  b  is  also  connected  to  a  bulb  C,  and  across 
the  two  capillaries  a  and  b  is  inserted  the  differential  manometer 
D.  The  bulbs  C  and  B  for  most  exact  work  should  be  inclosed 
in  a  bath  at  constant  temperature,  as  an  ice  bath.  The  relative 
sizes  of  the  bulbs  for  the  greatest  accuracy  will  depend  upon  the 
temperature  range  to  be  studied.  When  equilibrium  and  com- 
pensation are  established  at  any  temperature,  the  mass  of  the 
gas  in  the  two  parts  of  the  apparatus  will  be  the  same  if  the 
pressures  are  adjusted  to  equality  as  shown  by  the  sensitive 
manometer  D,  this  supposing  that  C  and  B  are  at  exactly  the 


GAS  PYROMETER 


73 


Fig.  9.    Callendar's  Thermometer. 

same  temperature.  For  a  change  in  temperature  the  volume 
change  of  the  gas  in  B,  i.e.,  forced  over  from  A,  may  be  made  by 
reading  this  volume  on  the  burette,  or  better  by  weighing  the 
displaced  mercury.  The  upper  stopcock  serves  to  exhaust  and 
fill  the  apparatus. 


74  HIGH  TEMPERATURES 

A  determination  of  temperature,  making  no  allowance  for 
correction  terms,  is  made  as  follows:  For  the  compensating  side 
of  the  apparatus  we  have 

VQ  =  volume          of  gas  in  C; 
mo  =  mass  of  gas  in  C; 

00  =  temperature  of  gas  in  C  on  gas  scale; 
po  =  pressure          of  gas  in  C; 
v  =  volume  of  capillaries; 
6  =  average  temperature  of  capillaries. 
Then 


where  k  is  a  constant. 

For  the  thermometer  proper  we  have,  using  a  similar  notation, 


the  subscript  t  referring  to  A,  and  m  to  B. 

But  m\—m  and  pt  =  po  as  conditions  of  compensation;  there- 
fore 


But  C  and  B  are  at  the  same  temperature,  00,  or  0m  =  00. 
Finally 

-  F<  X  0o 

*  yn  —  v 

'   m 

This  type  of  thermometer  with  an  air-filled  porcelain  bulb 
was  used  by  Callendar  and  Griffiths  to  determine  the  boiling 
point  of  sulphur,  for  which  temperature,  after  correcting  for  the 
expansion  of  porcelain,  they  obtained  444.55°  C.  on  the  constant- 
pressure  air  scale.  Eumorfopoulos,  using  air  in  a  bulb  of  Jena  i6ni 
glass,  has  obtained  very  recently  with  the  same  type  of  thermom- 
eter 444.55°,  with  a  range  in  eleven  experiments  of  0.37°  C.,  the 
thermometer  bulb  of  90  c.c.,  properly  screened,  being  put  into 
the  sulphur  vapor.  A  preliminary  publication  gave  443.58°, 


GAS   PYROMETER  75 

but  this  was  in  terms  of  an  uncertain  extrapolation  of  the  abso- 
lute expansion  of  mercury  from  100°,  which  was  used  to  obtain 
the  coefficient  of  expansion  of  the  glass  bulb  to  the  S.B.P.  The 
correction  of  +0.97°  was  computed  by  Callendar  and  Moss  in 
terms  of  their  very  recent  measurements  of  the  absolute  expan- 
sion of  mercury  to  300°  C. 

Eumorfopoulos  gives  also  the  exact  formulae  for  the  use  of 
such  a  thermometer.  He  found  Jena  i6HI  to  give  very  trouble- 
some changes  of  zero,  the  bulb  changing  in  volume  by  about 
i  per  cent  during  the  course  of  his  experiments. 

Used  with  a  quartz-glass  or  platinum-alloy  bulb,  such  a  gas 
thermometer  may  become  an  instrument  of  the  greatest  accu- 
racy for  the  experimental  extension  of  the  gas  scale  at  constant 
pressure. 

Holborn  and  Valentiner.  —  The  need  of  extending  the  gas 
scale  to  as  high  temperatur.es  as  possible  with  modern  appliances 
was  appreciated  at  the  Reichsanstalt,  and  this  difficult  task  was 
first  undertaken  in  1906  by  Holborn  and  Valentiner,  who  com- 
pared the  constant- volume  nitrogen  scale  to  1600°  C.  with  that 
of  the  platinum-rhodium  thermocouple  and  the  optical  pyrom- 
eter. 

The  experiments  were  executed  with  two  bulbs,  one  of  a 
20  per  cent  iridium  alloy  of  platinum  of  208  c.c.  capacity, 
heated  in  an  Heraeus  platinum-foil  resistance  furnace,  and  one 
of  iridium,  54  c.c.  capacity,  heated  in  an  Heraeus  indium-tube 
furnace.  Initial  pressures  of  136  to  250  mm.  were  used.  To 
avoid  contamination  of  the  wires  of  the  single  thermocouple  used, 
they  were  inclosed  in  quartz-glass  tubes.  In  spite  of  very  con- 
siderable lack  of  uniformity  of  temperature  within  the  furnace 
along  the  thermometer  bulb,  —  as  much  as  60°  C.  in  some  cases, 
—  and  very  considerable  corrections  for  the  dead  space,  — 125° 
to  150°  at  1600°  with  the  iridium  bulb,  —  these  observers  consider 
their  results  accurate  at  10°  C.  at  the  highest  temperatures. 
We  shall  return  to  their  thermoelectric  and  optical  measurements 
in  their  respective  chapters. 

Day,  Clement,  and  Sosman.  —  Not  since  the  classic  researches 


76  HIGH  TEMPERATURES 

of  Barus  in  the  early  eighties  have  serious  gas-thermometer 
investigations  been  carried  out  in  America  until  these  experi- 
menters (1904-10)  undertook  the  redetermination  of  a  series  of 
fixed  points  from  zinc  to  palladium  in  terms  of  the  constant- 
volume  nitrogen  thermometer.  The  preliminary  work  of  Day 
and  Clement  was  done  with  a  platinum-iridium  bulb,  but  on 


Fig.  10.     Bomb  Furnace  and  Gas  Thermometer  of  Geophysical  Laboratory. 

account  of  the  impossibility  of  completely  eliminating  the  con- 
taminating effects  of  evaporated  iridium  on  the  thermocouples, 
even  when  the  latter  were  quartz  inclosed,  a  bulb  of  80  Pt- 
20  Rh  was  substituted  in  the  later  experiments,  it  having  been 
shown  by  Holborn  and  Austin  that  rhodium  distills  from  the 
alloy  less  readily  in  nitrogen  than  does  iridium. 


GAS   PYROMETER 


77 


The  greatest  attention  was  given  to  the  perfecting  of  experi- 
mental methods  and  details  of  measurement,  such  as  (i)  an 
absolutely  gas-tight  bulb  of  constant  volume,  secured  by  having 
the  same  gas  and  pressure  outside  as  within  the  thermometer 
bulb,  which  necessitated  the  conversion  of  the  furnace  into  a  gas- 
tight  bomb  such  as  shown  in  Fig.  10;  (2)  uniform  distribution  of 
temperature  over  the  bulb  during  the  measurements,  obtained 
by  the  use  of  conveniently  spaced  and  independently  controlled 
heating  coils  of  platinum  wire  and  suitably  adjusted  diaphragms, 
as  shown  in  Figs,  n  and  12,  the  latter  being  improvements  over 


Figs,  n,  12.     Forms  of  Bulbs  and  Methods  of  Winding  and  Diaphragming. 

the  earlier  forms  of  furnace  used  at  the  Geophysical  Laboratory; 
(3)  the  reduction  to  a  minimum  of  the  error  due  to  the  dead  space 
in  the  capillary  tube  connecting  the  bulb  and  manometer,  the 
details  of  which  are  shown  in  Fig.  13;  (4)  the  exact  determination 
of  the  coefficient  of  expansion  of  the  bulb;  and  (5)  the  equalizing 
of  the  temperature  of  the  manometer  by  air  circulation.  In  the 
work  of  Day  and  Sosman,  a  return  was  made  to  the  Barus  form 
of  reentrant  bulb  (Figs,  n  and  12),  and  the  temperature  distribu- 


HIGH  TEMPERATURES 


Hg.Feed 
To  dry  bottle 


Fig.  13.     Manometer  of  Geophysical  Laboratory  Thermometer. 


GAS  PYROMETER  79 

tion  over  the  bulb  was  studied  by  means  of  numerous  attached 
platinum  wires,  which  were  used  differentially,  with  the  bulb  itself 
as  one  element  of  a  thermocouple,  as  well  as  by  independent 
couples.  The  preliminary  values  of  the  silver,  gold,  and  copper 
fixed  points  published  by  Day  and  Clement  were  later  found  by 
Day  and  Sosman  to  be  low,  mainly  on  account  of  incomplete 
temperature  compensation  along  the  bulb,  especially  at  the  ends. 
Day  and  Sosman  repeated  the  earlier  work  to  1100°  and  then 
determined  several  new  fixed  points,  salts  as  well  as  metals,  in  the 
interval  1100°  to  1600°,  and  consider  their  final  results  to  be  of 
great  accuracy,  — 0.3°  at  the  zinc  and  2°  at  the  palladium  point, 
—  after  an  exhaustive  study  of  some  twenty-five  possible  sources 
of  error.  For  measurements  at  1500°  C,  they  found,  for  example, 

7) 

the  errors  in  the  effect  of  the  dead  space  —  reduced  to  0.5°,  of 

temperature  integration  over  the  bulb  to  1.0°,  and  of  the  expan- 
sion coefficient  of  the  bulb  to  0.2°.  It  seems  safe  to  say  that 
their  results  are  certain  to  1.2°  at  400°  and  to  10°  at  1500°  C. 

It  should  be  noted,  however,  that  this  gas  thermometer  was 
designed  primarily  for  great  accuracy  at  the  highest  tempera- 
tures, and  as  used  lacks  sensibility  at  the  lower  temperatures,  so 
that  the  values  obtained  for  the  lower  fixed  points  would  seem  to 
have  less  relative  weight  than  for  the  higher  ones.  For  example, 
the  results  of  Day  and  Sosman,  according  to  Waidner  and  Bur- 
gess, from  interpolation  with  the  resistance  thermometer  from  the 
zinc  point,  would  lead  to  a  value  of  the  sulphur  point  i°  C.  lower 
than  the  value  directly  observed  by  several  experimenters.  The 
gas  and  resistance  thermometer  measurements  of  Holborn  and 
Henning  (1911)  confirm  those  of  Waidner  and  Burgess  on  these 
lower  freezing  points.  We  shall  return  to  this  question  in  the 
chapter  on  standardization.  There  is  no  doubt  that  this  inves- 
tigation of  Day  and  Sosman,  however,  is  the  best  that  has  yet 
been  made  in  gas  thermometry  at  the  highest  temperatures. 

Comparison  of  Results.  —  It  may  be  of  interest  at  this  point 
to  compare  some  of  the  characteristic  constants  and  numerical 
results  obtained  from  the  most  recent  observations,  using  the 


80 


HIGH  TEMPERATURES 


constant-volume  nitrogen  thermometer  at  high   temperatures. 
The  errors  are  those  assigned  by  the  observers. 

SOME  CONSTANTS  AND  RESULTS  WITH  THE  NITROGEN 
THERMOMETER. 


•j 

Wo 

*. 

o  . 

"3 

— 

^§ 

§^ 

$K 

S3 

B 

^u 

«  ~ 

fc« 

Freezing  Points. 

Observers. 

P. 

s 

V 

ft 

J°8 

IS 

•tJ  O 

g| 

J3 

3 

t  M 

ri| 

P  u 

5  ^ 

t-H 

eq 

> 

<s 

il 

o 

0  3 
^rt 

t> 

Zn 

Au 

Pd 

c.c. 

Holborn  and  Day  j 

286 
276 

80  Pt  •  20  Ir 
90  Pt  •  10  Ir 

208 

196 

.0042 
.0046 

(20 

43 

3  to  10 

(419.0 
1  ±0.5 

1064.0 

±1.0 

}.... 

Jaquerod  and          ( 
Perrot  ( 

195 

to  230 

|  Quartz  glass 

43 

.0180 

70 

3 

2 

I 

1067.2 
±1.8 

J.... 

Holborn  and           1 
Valentiner  ) 

147 
137 

80  Pt  .20  Ir 
Iridium 

208 
54 

0042 

022 

20 
(    90  to 
(  110 

35) 
J30J 

3  to  60 

|  1575 
1  ±10 

Day  and  Sosman  .  | 

217  to 
347 

J  80  Pt  •  20  Rh 

206 

OOI5 

3  to  6 

46 

I 

(418.2 
1  ±0.3 

1062  4 
±0.8 

1549.2 

±2.0 

Suggestions  for  Future  Experiments.  —  It  is  perhaps  easier  to 
criticize  than  execute  experiments  of  precision,  but  from  what 
has  been  said  above  it  is  evident  that  there  is  still  need  for  more 
work  with  the  gas  thermometer  before  the  high-temperature 
scale  is  established  in  an  entirely  conclusive  manner.  Thus,  the 
outstanding  uncertainty  of  nearly  0.5°  at  the  sulphur  boiling 
point  should  be  eliminated;  and  while  there  is  good  agreement, 
better  than  5°  at  1100°  C.,  a  difference  of  25°  between  observers 
exists  at  the  palladium  melting  point  (1550°  to  1575°) ;  and  at  tem- 
peratures to  which  it  now  seems  hopeless  to  extend  the  gas  scale 
directly,  this  range  of  uncertainty  increases,  becoming  about  100° 
at  3000°  C.,  or  at  the  melting  point  of  tungsten. 

Methods.  —  The  constant- volume  method  has  been  preferred 
by  almost  all  experimenters  who  have  worked  at  high  tempera- 
tures, and  the  results  by  this  method  also  have  smaller  correc- 
tions to  reduce  to  the  thermodynamic  scale.  For  the  lower  range 
of  temperatures  at  least,  in  view  of  the  outstanding  discrepancies, 
it  would  be  well  to  adopt  the  same  instrument  for  use  both  at 
constant  pressure  and  constant  volume.  To  as  high  tempera- 


GAS  PYROMETER  8l 

tures  as  possible,  the  bulb  should  be  immersed  in  stirred  liquid 
baths  to  insure  uniformity  of  temperature;  and  in  this  range  of 
temperatures,  or  to  perhaps  900°,  the  transfer  of  the  gas  scale 
can  probably  be  made  with  the  greatest  accuracy  by  means  of 
platinum-resistance  thermometers,  the  wires  of  which  can  be 
made  to  integrate  very  exactly  the  bulb  temperature.*  The  volu- 
menometric  method  has  not  been  used  in  any  recent  work, 
although  it  appears  to  possess  the  smallest  instrumental  cor- 
rections. It  labors  under  the  disadvantage  of  having  an  uncer- 
tain thermodynamic  correction,  but  this  largely  disappears  at 
high  temperatures,  where  the  outstanding  uncertainties  largely 
exceed  this  small  correction.  It  would,  therefore,  be  worth  while 
carrying  out  new  experiments  by  this  method,  especially  at  very 
high  temperatures.  The  method  of  Crafts  and  Meier  (p.  83)  is 
also  worthy  of  further  study  at  high  temperatures.  The  work 
of  Day  and  Sosman  shows  that,  for  the  constant-volume  ther- 
mometer, the  deformation  of  the  bulb  may  be  eliminated,  and 
the  error  due  to  the  dead  space  reduced  to  an  almost  negligible 
amount.  Their  work  also  shows  the  importance  of  an  exact 
determination  of  the  coefficient  of  expansion  of  the  bulb  and  an 
exact  adjustment  of  temperature  over  it  by  the  use  of  properly 
designed  electric  furnaces.  The  uncertainty  in  the  temperature 
of  the  manometric  parts  of  the  apparatus  gives  rise  to  an  appre- 
ciable error  which  may  be  eliminated  in  future  work  by  water- 
jacketing. 

The  Bulb.  —  All  recent  work  has  shown  the  superiority  of  the 
met^al  bulb  when  its  coefficient  of  expansion  is  carefully  deter- 
mined. The  alloy  80  Pt  •  20  Rh  is  the  material  that  has  been 
used  so  far  which  best  suits  all  the  requirements  for  temperatures 
to  1600°  C,  namely,  rigidity,  impermeability,  regular  expansion, 
and  small  contaminations  of  the  auxiliary  temperature  apparatus. 
The  best  form  appears  to  be  cylindrical  with  a  reentrant  tube. 
It  may  be  possible  to  find  refractory  earths  which  are  sufficiently 
impermeable  to  use  with  some  modification  of  the  Crafts  and 

*  Both  of  these  improvements  and  others  have  been  introduced  by  Holborn 
and  Henning  to  450°  C.  since  the  above  was  written. 


82  HIGH  TEMPERATURES 

Meier  method  at  very  high  temperatures;  or  possibly  metallic 
tungsten  or  one  of  its  alloys  may  be  adopted  for  use  in  a  suitable 
atmosphere  for  pushing  the  gas  scale  to  the  highest  limits.  In 
all  cases  it  is  desirable  to  have  the  volume  of  the  bulb  as  great  as 
possible  consistent  with  uniform  temperature  distribution.  To 
500°,  there  should  be  no  difficulty  in  using  a  500  c.c.  bulb. 

The  Gas.  —  Nitrogen  has  proved  satisfactory  in  every  respect, 
and  this  gas  will  probably  be  continued  in  use,  although  there 
would  be  some  theoretical  advantage,  at  least  for  the  higher 
temperatures,  in  substituting  one  of  the  monatomic  inert  gases, 
such  as  argon  or  helium. 

It  is  questionable  whether  it  is  worth  while  to  attempt  to  ex- 
tend the  use  of  the  gas  thermometer  above  1600°  C.,  as  the  con- 
stants of  the  laws  of  radiation  can  be  exactly  determined  in  this 
range,  and  the  radiation  laws  are  eminently  suited  for  extrapola- 
tion, as  they  give  the  thermodynamic  scale  directly. 

The  Manometer.  —  It  would  be  well  to  eliminate  the  some- 
what troublesome  and  uncertain  reference  to  a  variable  pressure, 
that  of  the  atmosphere,  by  the  entire  elimination  of  the  barom- 
eter. This  can  be  done,  whatever  the  type  of  gas  thermometer 
used,  by  evacuating  to  zero  pressure  the  space  above  the  manom- 
eter column  and  sealing  off  the  manometer  tube  provided  with  a 
suitable  globe  or  bulb  at  the  top.*  Variations  in  the  tempera- 
ture of  the  mercury  columns  of  the  manometer  may  be  com- 
pletely eliminated  by  water-jacketing.  The  errors  of  the  ma- 
nometer are  then  easily  made  negligible  compared  with  those  of 
expansion  of  the  bulb,  the  temperature  distribution  over  it,  and 
the  transfer  to  the  comparison  thermometer. 

In  general,  it  may  be  stated  that  it  is  not  worth  while  to  carry 
out  any  further  gas  thermometer  experiments  unless  the  utmost 
precautions  are  taken  to  assure  the  highest  accuracy  possible 
with  modern  appliances. 

Industrial  Air  Pyrometers.  —  There  have  been  attempts  to 
construct  air  thermometers  suitable  for  industrial  usage,  the 
argument  sometimes  being  advanced  that  a  gas  pyrometer  is 

*  See  note,  p.  81. 


GAS   PYROMETER  83 

per  se  better  than  any  other.  As  we  have  seen,  however,  there 
is  probably  no  physical  instrument  which  is  more  difficult  to 
employ  satisfactorily,  and  any  seeming  gain  in  making  direct 
use  of  an  air  thermometer  for  industrial  use  is  wholly  illusory. 
Other  evident  objections  are  fragility,  uncertain  correction  due 
to  the  dead  space,  and  the  development  of  small  and  often 
unperceived  leaks.  Furthermore,  an  empirical  calibration  is 
necessary,  so  that  such  an  instrument  does  not  carry  the  gas 
scale  about  with  itself. 

Among  the  instruments  that  have  been  considerably  used  is 
Wiborgh's   air  pyrometer,  shown  in  Fig.    14.      A   lens-shaped 


WIBOR6H  AIR 
PYROMETER 


Fig.  14. 

reservoir  V'  is  open  to  the  air  before  an  observation  is  taken,  but 
when  a  temperature  is  to  be  read  this  lens  is  closed  to  the  outer 
air  and  collapsed  by  a  lever  Z,,  thus  adding  a  definite  mass  of 
air  to  the  bulb  V  of  the  thermometer;  the  resulting  pressure  is 
transmitted  to  a  dial  as  in  an  aneroid  barometer;  provision  is 
made  for  automatically  correcting  for  variations  in  the  pressure 
and  temperature  of  the  atmosphere.  The  Bristol  Company  have 
also  made  industrial  forms  of  gas  pyrometer. 

Indirect  Processes.  —  We  shall  place  in  this  list  various  experi- 
ments in  which  the  laws  of  the  expansion  of  gases  have  been  used 
only  in  an  indirect  way,  or  have  been  extended  to  vapors. 

Method  of  Crafts  and  Meier.  —  It  is  a  variation  of  the  method 
of  H.  Sainte-Claire-Deville  and  Troost,  consisting  in  removing 
the  gas  by  means  of  a  vacuum.  Crafts  and  Meier  displaced  the 


84  HIGH  TEMPERATURES 

gas  of  the  pyrometer  by  carbonic  acid  or  hydrochloric  acid,  gases 
easily  absorbable  by  suitable  reagents.  Hydrochloric  acid  is  the 
more  convenient,  for  its  absorption  by  water  is  immediate;  but 
there  is  to  be  feared  at  high  temperatures  its  action  on  the  air 
with  formation  of  chlorine ;  it  is  preferable  to  employ  nitrogen  in 
place  of  air. 

The  apparatus  (Fig.  15)  consists  of  a  porcelain  bulb,  whose 
inlet  is  large  enough  to  let  pass  the  entrance  tube  of  the  gas,  which 
reaches  to  the  bottom  of  the  bulb.     This 
arrangement  increases  considerably  the  in- 
[\  p  fluence  of  the  dead  space  and  consequently 

diminishes  the  precision  of  the  determina- 
tions. 

This  method  is  especially  convenient  for 
observations  on  the  densities  of  vapors  which 
are  made  by  the  same  apparatus;  it  then 
allows  of  having  an  approximate  idea  of  the 
temperatures  at  which  the  experiments  are 
made. 

Crafts  and  Meier  have  in  this  way  deter- 
Fig.  15.    Method  of     ^^(1  the  variations  in  density  of  iodine 

Crafts  and  Meier. 

vapor  as  a  function  of  the  temperature. 
Regnault  had  previously  proposed  a  similar  method,  without, 
however,  making  use  of  it. 

1.  One  fills  with  hydrogen  an  iron  vessel  brought  to  the 
temperature  that  one  desires  to  measure,  and  the  hydrogen  is 
driven  out  by  a  current  of  air;  at  the  outlet  of  the  metallic 
reservoir  the  hydrogen  passes  over  a  length  of  red-hot  copper, 
and  the  water  formed  is  absorbed  in  tubes  of  sulphuric  acid  in 
pumice  stone  and  weighed.    This  method,  very  complicated,  is 
bad  on  account  of  the  permeability  of  the  iron  at  high  tempera- 
tures. 

At  the  same  time,  he  proposed  the  following  method: 

2.  An  iron  bottle  containing  mercury  is  taken;  the  vessel, 
being  incompletely  closed,  is  heated  to  the  desired  temperature 
and  then  allowed  to  cool,  and  the  remaining  mercury  is  weighed. 


GAS  PYROMETER  85 

This  method  is  also  defective  on  account  of  the  permeability 
of  iron  at  high  temperatures;  the  hydrogen  of  the  furnace  gases 
can  penetrate  to  the  inside  of  the  recipient  and  drive  out  an 
equivalent  quantity  of  mercury  vapor. 

Methods  of  H.  Sainte-Claire-Deville.  —  i.  This  savant  tried 
in  the  first  place  to  measure  temperature  by  a  process  analogous 
to  that  of  Dumas'  determination  of  vapor  densities.  He  took  a 
porcelain  bulb  full  of  air,  and  heated  it  in  the  inclosure  whose 
temperature  was  wanted,  and  sealed  it  off  by  the  oxyhydrogen 
flame.  He  measured  the  air  remaining  by  opening  the  bulb  under 
water  and  weighing  the  water  that  entered,  or  else  he  determined 
merely  the  loss  in  weight  of  the  bulb  before  and  after  heating. 
Observations  taken  on  the  boiling  point  of  cadmium  gave  860°. 

2.  In  a  second  method,  which  has  the  advantage  of  replacing 
the  air  by  a  very  heavy  vapor,  Deville  returned  to  the  idea  of 
Regnault,  consisting  in  utilizing  the  vapor  of  mercury;  but  he 
ran  against  a  practical  difficulty.  He  had  replaced  the  permeable 
iron  recipients  by  porcelain  recipients;  the  mercury  condensed 
in  the  neck  of  the  pyrometer  and  fell  back  in  cold  drops  which 
caused  the  bulb  to"  break. 

For  this  reason  he  abandoned  mercury  and  replaced  it  with 
iodine;  the  return  of  a  cold  liquid  was  completely  obviated  by 
reason  of  the  nearness  of  the  boiling  point  of  this  substance  (175°) 
and  its  fusing  point  (i  13°).  A  large  number  of  observations  were 
made  by  this  method;  the  boiling  point  of  zinc,  for  example,  was 
found  to  be  equal  to  1039°. 

This  method  is  quite  faulty,  as  the  iodine  does  not  obey  the 
laws  of  Mariotte  and  Gay-Lussac.  The  vapor  density  of  this 
substance  decreases  with  rise  of  temperature,  this  effect  being 
attributed  to  a  doubling  of  the  iodine  molecule.  This  fact  was 
established  by  Crafts  and  Meier  and  confirmed  by  Troost. 

Method  of  D.  Berthelot.  —  All  the  preceding  methods  are 
limited  by  the  difficulty  of  realizing  solid  envelopes  resisting 
temperatures  higher  than  1600°.  D.  Berthelot  has  devised  a 
method  which,  at  least  in  theory,  may  be  applied  to  any  tempera- 
tures, however  high,  because  there  is  no  envelope  for  the  gas,  or 


86  HIGH  TEMPERATURES 

at  least  no  envelope  at  the  same  temperature.  It  is  based  on 
the  variation  of  the  index  of  refraction  of  gaseous  mass  heated 
at  constant  pressure;  the  velocity  of  light  depends  upon  the 
chemical  nature  and  the  density  of  this  medium,  but  is  indepen- 
dent of  its  physical  state.  A  gas,  a  liquid,  or  a  solid  of  the  same 
chemical  nature  produces  a  retardation  of  the  light  dependent 
only  upon  the  quantity  of  matter  traversed;  this  law,  sensibly 
true  for  any  bodies  whatever,  should  be  rigorously  exact  for 
substances  approaching  the  condition  of  perfect  gases.  This 
retardation  is  measured  by  the  displacement  of  interference 
fringes  between  two  beams  of  parallel  light,  the  one  passing 
through  the  cold  gas,  the  other  through  the  hot  gas.  In  reality, 
Berthelot  employs  a  null  method;  he  annuls  the  displacement  of 
the  fringes  in  changing  at  constant  temperature  the  pressure  of 
the  cold  gas  until  its  density  is  equal  to  that  of  the  gas  in  the 
warm  arm  which  is  at  constant  pressure. 

There  is  a  difficulty  arising  from  the  necessity  of  separating  the 
light  into  two  parallel  beams,  then  reuniting  them  without  im- 
parting a  difference  of  phase  which  renders  the  fringes  invisible 
with  white  light.  This  is  done  in  the  following  way  (see  Fig.  16) : 

A  beam  of  light  ab  falls  on  a  mirror  MM' ,  which  breaks  it  up 
into  two  parallel  beams,  bf  and  cd\  in  order  to  separate  the  beams 
so  as  to  be  able  to  place  apparatus  conveniently  with  respect  to 
them,  a  prism  P  gives  to  the  beam  bf  the  direction  gh\  one  can 
thus  secure  a  separation  of  92  mm.  A  second  prism  PI  brings 
the  beam  cd  into  Im,  and  after  reflection  from  a  second  mirror, 
MiMi,  the  fringes  are  observed  in  a  telescope  focused  for  parallel 
rays.  The  tubes  containing  the  gases  are  placed  at  T  and  TV 

It  is  evidently  necessary  that  the  prisms  P  and  PI  be  perfectly 
made.  A  preliminary  adjustment  is  made  with  yellow  light, 
then  it  is  perfected  with  white  light. 

The  tube  at  variable  pressure  is  closed  by  two  pieces  of  plate 
glass,  as  is  also  the  warm  tube;  these  four  plates  should  be  abso- 
lutely alike.  The  warm  tube  is  heated  by  a  vapor  bath  at  low 
temperatures,  by  an  electric  current  passing  through  a  spiral  at 
high  temperatures. 


GAS   PYROMETFJl  87 

But  there  is  a  difficulty  in  that  in  the  warm  tube  there  exists 
a  region  of  variable  temperature  between  the  warm  zone  and  the 
cold  atmosphere. 

To  eliminate  the  influence  of  this  variable  zone,  there  are  inside 
the  warm  tube  two  tubes  containing  running  cold  water,  whose 
distance  apart  can  be  changed;  it  is  assumed  that  the  variable 
region  remains  the  same,  and  that  distance  between  the  two 
tubes  gives  the  warm  column  actually  utilized.  It  follows  that 


T 

4 

d 

.   <& 

m 

\ 

9 

J 

p 

1 

9 

I* 

: 

-    Tfc 

Fig.  1 6.     D.  Berthelot's  Method. 

the  comparative  lengths  of  the  warm  column  and  of  the  cold 
column  (this  latter  remaining  constant)  are  not  the  same;  the 
formula  to  be  used  will  be  somewhat  more  complicated. 
n  being  the  index  of  refraction  of  a  gas  and  d  its  density,  we 

have 

n  —  i  =  kd. 

In  the  constant-pressure  tube 

*L  =L. 

do      po 

To  obtain  the  invariability  of  the  fringes,  it  is  necessary  that 

(HI  —  n0)  L  =  (nf  —  n0)  I, 


88  HIGH  TEMPERATURES 

L  being  the  length  of  the  cold  tube,  and  /  the  displacement  of 
the  warm  tube. 

T      /  J  7    \       J-  I      /If  1    \     1 

K  \Q,\  —  UQ)  Li   =  K  (d     —  ttQ/  '> 

ifc-«V-«£- 


an  expression  which  gives  a  relation  between  the  pressures  and 
the  temperatures. 

This  method,  employed  for  the  control  of  the  boiling  points, 
has  given  the  following  results: 

Pressure  Temperature       Temperature 

observed.  calculated. 

Alcohol 74i.5mm.          77.69°  77.64° 

Water 740. i  99.2  99.20 

Water 761.04  100.01  100.01 

Aniline 746 . 48  183 . 62  183 . 54 

Aniline 760.91  184.5  184.28 

Berthelot  has  standardized  by  the  same  method  thermocouples 
which  he  used  to  determine  the  fusing  points  of  silver  and  gold, 
and  the  boiling  points  of  zinc  and  cadmium: 

Silver,  freezing 962  °  C. 

Gold,  freezing 1064 

Zinc,  boiling 920 

Cadmium,  boiling 778 

The  numbers  found  are  nearly  identical  with  those  which 
result  from  the  best  determinations  made  by  other  methods. 

We  shall  discuss  further  the  determinations  of  fixed  points  in 
pyrometry  in  Chapter  XI. 


CHAPTER  III. 
CALORIMETRIC  PYROMETRY. 

Principle.  —  A  mass  m  of  a  body,  brought  to  a  temperature  T, 
is  dropped  into  a  calorimeter  containing  water  at  a  temperature 
/o.  Let  ti  be  the  final  temperature  of  water  and  substance.  M 
being  the  water  equivalent  of  the  substances  in  contact  (water, 
calorimetric  vessel,  thermometer,  etc.)  which  are  raised  from  tQ 
to  ti,  Lt  the  heat  required  to  warm  unit  mass  of  the  body  from  ti 
to  r,  we  have 

LTt  X  m  =  M(h  -  t0). 

Taking  as  origin  of  temperatures  the  zero  of  the  centigrade  ther- 
mometer, the  heat  required  to  warm  unit  mass  of  the  body  to 
the  temperature  T  will  be 

The  quantity  L0  is  easy  to  calculate,  because  the  specific 
heats  at  low  temperatures  are  sufficiently  well  known: 

The  expression  for  the  total  heat  becomes 


m 

ti  and  /o  are  the  temperatures  given  by  the  direct  readings  of  the 
thermometer. 

The  value  of  the  second  member  is  thus  wholly  known,  and 
consequently  that  of  the  first  member  which  is  equal  to  it.  If 
previous  experiments  have  made  known  the  value  of  the  total 
heat  LQ  for  different  temperatures,  one  may  from  the  knowledge 
of  LQ  determine  the  value  of  T.  It  will  be  sufficient  to  trace  a 
curve  on  a  large  scale  whose  ordinates  are  temperatures,  and 

89 


90 


HIGH  TEMPERATURES 


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TOTAL  HEAT  IN  CALORIES 
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CALORIMETRIC  PYROMETRY  91 

abscissas  total  heats,  and  to  find  upon  this  curve  the  point  whose 
abscissa  has  the  value  given  by  the  calorimetric  experiment. 

In  Fig.  17  are  given  curves  of  temperature  in  terms  of  total 
heat  from  o°  C.  for  the  several  metals  used  in  specific  heat  py- 
rometry.  The  values  of  the  total  heats  are  the  means,  for  each 
metal,  of  the  experimental  results  cited  in  the  several  tables 
which  follow. 

Choice  of  Metal.  —  Four  metals  have  been  proposed :  platinum, 
iron,  nickel,  and  copper. 

Platinum.  —  This  metal  was  first  proposed  by  Pouillet,  and 
taken  up  again  by  Violle.  It  is  much  to  be  preferred  to  the  other 
metals;  its  total  heat  has  been  compared  directly  with  the  indi- 
cations of  the  gas  thermometer.  This  metal  can  also  be  repro- 
duced identical  with  itself.  Iridium,  which  commercial  platinum 
often  carries,  has  about  the  same  specific  heat.  The  high  price 
of  these  substances  is  an  obstacle  to  their  use  extensively  in 
works ;  for  a  calorimeter  of  a  liter  it  is  necessary  to  have  at  least 
loo  grm.  of  platinum,  —  or  $100  in  a  volume  of  5  c.c.,  —  easily 
lost  or  made  away  with. 

Violle  determined  the  total  heat  of  platinum  from  o°  to  1 200°, 
and  computed  it  by  extrapolation  to  1800°. 

W.  P.  White  has  determined  the  specific  heat  of  platinum 
to  1500°,  obtaining  somewhat  lower  values  than  Violle.  The 
differences  cannot  be  accounted  for  by  differences  in  tempera- 
ture scales.  A  few  measurements  by  Tilden  to  600°  give  values 
between  the  others,  and  Plato  at  600°  and  750°  finds  values 
agreeing  closely  with  White's. 

TOTAL  HEAT  OF  PLATINUM  FROM  o°  C.  IN  CALORIES. 


Temperature. 

Violle. 

White. 

Temperature. 

Violle. 

White. 

IOO 

3-23 

IOOO 

37-70 

35-45 

2OO 

6.58 

I  IOO 

42.13 

39-32 

300 

9-95 

9-49 

1200 

46.65 

43-28 

4OO 

13-64 

13.09 

1300 

(5I-35) 

47-26 

500 

17-35 

16.75 

1400 

(56.14) 

49-22 

600 

21.  18 

20.44 

1500 

(61.05) 

55-20 

700 

25-13 

24-15 

1600 

(66.08) 

(59-26) 

800 

29.20 

27.88 

1700 

(7I-23) 

(63.45) 

QOO 

33-39 

31-63 

1800 

(76.50) 

92  HIGH  TEMPERATURES 

White's  measurements  on  mean  specific  heat  satisfy  the 
equation  0.03193  +  3.4  •  io~6t. 

Iron.  —  Regnault,  in  an  investigation  made  for  the  Paris  Gas 
Company,  had  proposed,  and  caused  to  be  adopted,  iron,  in 
attributing  to  it  a  specific  heat  of  0.126,  instead  of  0.106  at  o°. 
He  used  a  cube  of  7  cm.  sides  which  was  thrust  into  the  furnaces 
by  means  of  long  iron  bars.  The  calorimeter  was  of  wood  and 
had  a  capacity  of  4  liters. 

Various  observers  have  determined  the  total  heat  of  iron;  at 
high  temperatures  the  accord  is  not  perfect  among  the  results, 
as  shown  in  the  following  table: 

TOTAL  HEAT  OF  IRON  FROM  o°  C.  IN  CALORIES. 


Temperature. 

Pionchon. 

Euchene. 

Harker. 

Oberhoffer. 

weiss  an 
Beck. 

IOO 

ii  .0 

II  .O 

200 

22.5 

23.0 

23.0 

23-4 

23.1 

300 

36.8 

37-0 

37-0 

37-5 

36.1 

400 

51-6 

•52.0 

51-3 

52.4 

49-5 

500 

68.2 

69-5 

66.9 

66.0 

64.4 

600 

87.0 

84.0 

83-8 

84.6 

81.2 

700 

108.4 

106.0 

104.1 

II3-4 

IOI.O 

800 

135-4 

131.0 

127.8 

135-2 

124.2 

900 

157-2 

151-5 

148.0 

152.1 

148.5 

IOOO 

170.9 

173-0 

155-7 

167.0 

I  IOO 

168.8 

182.6 

1200 



199.2 

1300 

215.8 

1400 

232.4 

1500 

250.5 

The  determinations  of  Pionchon  and  of  Euchene  are  in  terms 
of  incorrect  temperature  scales,  at  least  above  800°  C.,  and  al- 
though those  of  the  other  observers  are  in  terms  of  approxi- 
mately the  same  scale,  the  one  ordinarily  used  to-day,  this  agree- 
ment is  far  from  satisfactory.  Oberhoffer's  results  show  an 
abrupt  change  in  specific  heat  beginning  at  650°,  and  changes 
in  specific  heat  corresponding  to  the  allotropic  forms  of  iron. 
The  results  of  Weiss  and  Beck  show  an  abrupt  change  in  specific 
heat  at  750°,  corresponding  to  the  magnetic  transformation  point. 
According  to  Oberhoffer  and  Meuthen,  the  addition  of  carbon  to 
iron  increases  the  specific  heat  in  the  proportion  of  o.oon  per 
each  0.5  per  cent  carbon  added,  at  least  for  the  temperature 
range  o°  to  650°  C. 


CALORIMETRIC  PYROMETRY 


93 


In  spite  of  its  common  use  for  this  purpose,  this  metal  is  not 
at  all  suitable  for  calorimetric  use,  by  reason  in  the  first  place 
of  its  great  oxidability.  There  is  formed  at  each  heating  a 
coating  of  oxide  which  breaks  off  upon  immersion  in  water,  so 
that  the  mass  of  the  metal  varies  from  one  observation  to  the 
next.  Besides,  iron,  especially  when  it  contains  carbon,  possesses 
changes  of  state  accompanied  during  the  heating  by  a  marked 
absorption  of  heat.  By  cooling  in  water,  hardening  takes  place, 
which  may  irregularly  prevent  the  inverse  transformations. 
The  use  of  electrolytic  iron  is  therefore  preferable,  since  the  most 
marked  transformation  and  the  one  at  the  lowest  temperature 
is  thus  avoided,  and  the  oxidation  is  less. 

Nickel.  —  At  the  Industrial  Gas  Congress  in  1889  Le  Chatelier 
proposed  nickel,  which  is  but  slightly  oxidizable  up  to  1000°, 
and  which  above  400°  does  not  possess  changes  of  state  as  does 
iron. 

The  total  heat  of  nickel  has  been  determined  by  Pionchon,  by 
Euchene,  and  by  Weiss  and  Beck. 

The  differences  are  due  very  probably  in  part  to  impurities 
that  the  nickel  may  contain,  as  well  as  to  experimental  and 
temperature-scale  uncertainties. 

TOTAL   HEAT  OF   NICKEL  FROM  o°  C.    IN   CALORIES. 


Temperature. 

Pionchon. 

Euchene. 

Weiss  and 
Beck. 

100 

II  .O 

12  .O 

200 

22.5 

24.0 

23.1 

300 

42.0 

37-0 

36.2 

4OO 

52.0 

50.0 

50.0 

500 

65-5 

63.5 

63-2 

600 

78-5 

75-0 

76.6 

700 

92.5 

90.0 

90.0 

800 

107.0 

103.0 

104.9 

900 

123.0 

II7-S 

IOOO 

I38-S 

134-0 

Copper  is  sometimes  used,  and  although  when  pure  it  appears 
to  possess  no  transformation  regions,  it  oxidizes  and  scales  very 
readily  and  cannot  be  used  to  as  high  temperatures  as  any  of  the 


94 


HIGH  TEMPERATURES 


other  metals  proposed.  In  the  following  table  are  given  values 
of  the  total  heat  of  copper  as  computed  from  the  experiments 
of  Le  Verrier  and  of  Frazier  and  Richards. 

TOTAL  HEAT  OF  COPPER  FROM  o°  C.  IN  CALORIES. 


Temperature. 

Le  Verrier. 

Frazier  and 
J.  W.  Richards. 

100 
2OO 

10.4 

20.8 

9.6 
IQ-5 

300 

31.2 

29.8 

400 

42.2 

40.4 

500 

600 

54-7 
66.5 

51-4 
62.8 

700 
800 

77-8 
91.0 

74-4 
86.5 

900 

IOOO 

103.8 
115.6 

99.0 
111.7 

Calorimeters.  —  In  laboratories  a  platinum  mass  is  often 
employed  with  Berthelot's  calorimeter,  a  description  of  which  is 
given  in  various  publications  on  calorimetry  (Fig.  18).  The 

thermometer  used  for  the  meas- 
urement of  the  rise  in  temper- 
ature should  be  very  sensitive, 
so  that  a  rise  of  from  2°  to  4°  be 
sufficient  in  order  to  render  neg- 
ligible the  cooling  correction. 
If  use  is  made,  for  instance,  of 
a  thermometer  giving  the  hun- 
dredth of  a  degree,  the  mass  of 
platinum  should  be  about  one- 
twentieth  the  mass  of  the  water 
in  the  calorimeter. 

A  form  of  water-inclosed  cal- 
orimeter, with  furnace,  such  as 


Fig.  18.    Berthelot's  Calorimeter. 


used  by  White  in  specific  heat  determinations,  is  shown  diagram- 
matically  in  Fig.  19.  This  method  of  operation  is  also  appli- 
cable to  temperature  estimations.  The  water  cover  is  swung 
aside  when  the  platinum  mass  is  dropped  into  the  calorimeter. 


CALORIMETRIC   PYROMETRY 


95 


This  type  of  calorimeter  is  to  be  preferred  in  exact  calorimetric 
work  where  high  temperatures  are  involved,  as  the  uncertainties 
•of  radiation  and  evaporation  are  reduced  to  a  minimum.  The 

usual  mercury  thermometer  may 
be  replaced  to  advantage  by  some 
form  of  electric  thermometer. 

Industrial  Calorimeters  (Fig.  20). 
—  In  the  arts,  where  the  measure- 
ments are  made  with  less  precision, 
and  where  it  is  necessary  to  con- 
sider the  cost  of  installation  of  the 
apparatus,  nickel  may  be  made  use 
of,  a  thermometer  giving  tenths  of  a 
degree,  and  zinc  calorimeter,  which 
may  be  home-made.  Such  an  in- 
stallation may  cost  as  little  as  $5. 
A  mass  of  nickel  should  be  used 
equal  to  one-twentieth  of  the  mass 
of  water  of  the  calorimeter. 


Fig.  20.     Industrial  Calorimeter. 
Fig.  19.     White's  Calorimeter. 

The  calorimeters  used  by  the  Paris  Gas  Company  are  after 
the  Berthelot  pattern;  they  are  also  water- jacketed. 

Such  an  apparatus  may  consist  of  a  cylindrical  calorimeter  A 
of  two  liters  capacity,  of  zinc  or  of  copper;  a  double  cylindrical 
jacket  B  of  the  same  metal,  containing  water,  and  which  may  be 
surrounded  by  felt  on  the  outside.  The  calorimeter  rests  on  this 


96 


HIGH  TEMPERATURES 


n 


jacket  by  means  of  a  wooden  support  C.    There 
is  preferably  a  metallic  cover  in  good  contact 

Fig.  21.    Metal       with  the  outside  vessel.     A  thermometer  grad- 
Camer.  uated  to  fifths  of  a  degree,  having  a  small  but 

quite  long  bulb,  serves  as  stirrer.  The 
thermometric  substance  is  a  piece  of 
nickel  of  mass  equal  to  one-tenth  that 
of  the  water,  or  200  grm.,  so  as  to 
have  considerable  rise  of  temperature 
easy  to  read  by  the  workmen  who  make 
the  measurements. 

As  a  general  rule,  one  must  avoid 
placing  the  thermometric  substance 
upon  the  floor  of  the  furnace.  The 
piece  of  nickel,  which  is  made  in  the 
form  of  small  cylinders  having  from  15 
to  25  mm.  diameter  and  from  10  to  30 
mm.  length,  rests  so  as  to  be  insulated 
from  the  floor  in  a  nickel  crucible  pro- 
vided with  a  foot  and  with  two  arms 
attached  somewhat  above  the  center 
of  gravity.  When  it  has  been  heated 
for  a  half-hour,  an  observer  takes  out 
the  crucible  with  a  forked  rod,  and 
another  seizes  this  crucible  with  tongs 
to  empty  it  into  the  calorimeter. 

Use  is  not  made  of  an  iron  crucible 
because  this  metal  oxidizes  and  lets 
drop  scales,  which  falling  into  the  cal- 
orimeter would  vitiate  the  experiment. 
Fig.  21  shows  a  suitable  arrangement 
for  containing  a  nickel  cylinder. 

Siemens  Calorimeter.  —  A  convenient 
form  of  direct-reading  calorimeter  due 
to  Siemens  is  shown  in  Fig.  22.    Using    Fig' "'  Siemens Calorimeter- 
always  the  same  mass  of  water  and  a  ball  of  given  mass  and 


CALORIMETRIC  PYROMETRY  97 

kind,  the  thermometer  or  an  auxiliary  scale  may  be  graduated 
to  read  directly  the  temperature  attained  by  the  heated  ball. 
Hollow  copper  cylinders  are  usually  furnished  with  this  appa- 
ratus. 

Precision  of  Measurements.  —  Biju-Duval  made  a  series  of 
experiments  to  study  the  sources  of  error  arising  from  the  use  of 
the  industrial  calorimeter  by  comparing  its  indications  to  those 
of  the  thermoelectric  pyrometer  of  Le  Chatelier.  The  observa- 
tions were  taken  by  varying  the  following  conditions: 

Use  of  thermometer  graduated  to  |°  or  to  -£$°. 
Use  of  the  old  wooden  gas-works  calorimeter  or  of  the  water- 
jacketed  calorimeter. 
Use  of  iron  or  nickel. 

I.  Experiment.  —  Old  wooden  gas-works  calorimeter.  Iron. 
Thermometer  in  fifths. 

P  =  10,000  grm. 
p  =  1031  grm. 
/o  =  20.8° 
/i  =  36-2° 
Qo  =  J53-5  cal. 
Computed  temperature: 

Mean  specific  heat  of  iron  =  0.108  t  =  1420° 

Mean  specific  heat  of  iron  =0.126  /  =  1210 

Totai  heat  according  to  Biju-Duval  /  =    915 

Thermoelectric  pyrometer  £  =    970 

It  is  thus  evident  that  the  mean  specific  heats  even  with  the 
correction  suggested  by  Regnault  give  temperatures  much  too 
high.  With  the  curve  of  total  heats  the  temperature  found  is 
much  too  low  on  account  of  the  following  losses  of  heat: 

1.  Absorption  of  heat  by  the  wooden  walls; 

2.  Radiation  from  the  iron  cube  during  transfer; 

3.  Cooling  of  the  water  in  the  calorimeter,  whose  temperature 
exceeded  by  16°  the  temperature  of  the  surroundings. 

The  following  experiments  were  made  with  the  thermometer 


98  HIGH  TEMPERATURES 

reading  to  -fa0-,  the  piece  of  nickel  was  protected  against  radiation 
by  a  crucible.     The  two  calorimeters  were  compared. 

II.    Trial  with  the  Wooden  Calorimeter.  — 

T  =  975°  by  the  thermoelectric  pyrometer 
P  =  10,000  grm. 
p  =  145  grm. 

to  =  20.21° 


/I  =  21.99 
TQ 


LT  =  125  cal. 


L    —  131.5  cal.  from  the  curve  at  975°. 

The  difference  is  6.5  calories,  or  5  per  cent  loss  due  to  the 
jacket. 

III.    Trial  with  the  Water-jacketed  Calorimeter.  — 

T  =  985° 
P  =  2000  grm. 
p  =  48.4  grm. 
*o  =  18.86° 

\=  2I'95° 
L0  =  130  cal. 

rp 

LQ  =  133  cal.  from  the  curve  at  985°. 

The  difference  is  3  calories,  or  a  loss  of  about  2  per  cent  only 
when  use  is  made  of  a  carefully  made  calorimeter  and  of  a  ther- 
mometer giving  -g*Q°.  This  corresponds  to  an  uncertainty  of  less 
than  20°  in  the  temperatures  sought.  Some  of  this  uncertainty 
may  be  due  to  the  temperature  assumed  as  correct  in  these 
measurements  and  to  loss  of  heat  during  transfer.  It  is  possible 
to  work  to  better  than  5°  by  the  most  refined  methods,  using  a 
platinum  mass.  With  the  ^0°  thermometers,  necessitating  a 
much  greater  rise  of  temperature  of  the  water  in  the  calorimeter, 
an  uncertainty  of  25°  or  more  may  exist.  The  relatively  small 
mass  of  water  used  with  a  less  sensitive  thermometer  is  not 
necessarily  a  disadvantage,  however,  if  the  calorimeter  is  properly 
protected  against  heat  loss  and  evaporation  due  to  the  greater 
temperature  rise. 


CALORIMETRIC   PYROMETRY  99 

Conditions  of  Use.  —  The  advantages  of  the  calorimetric 
pyrometer  are: 

1.  Its  low  net  cost; 

2.  The  ease  of  its  use,  which  allows  of  putting  it  in  the  hands 
of  a  workman. 

Its  inconveniencies  are : 

1.  The  time  necessary  to  take  an  observation,  about  a  half- 
hour,  except  with  the  Siemens  form; 

2.  The  impossibility  of  taking  continuous  observations; 

3.  The  impossibility  of  exceeding  1000°  by  the  use  of  the  piece 
of  nickel ; 

4.  The  deterioration  of  the  balls  used  due  to  oxidation. 

Its  use  does  not  seem  to  be  recommendable  for  laboratories, 
as  there  are  continuous  methods  of  greater  accuracy  readily 
available  for  such  uses.  In  the  laboratory,  the  calorimetric 
method  is  used  ordinarily  for  the  determination  of  specific  heats 
at  high  temperatures  rather  than  of  these  temperatures.  In 
recent  years,  there  have  been  introduced  many  refinements  into 
calorimetric  measurements,  such  as  vacuum-jacketed  calorimeters 
which  nearly  eliminate  heat  losses  during  the  rise  in  temperature 
within  the  vessel;  resistance  thermometers  and  thermoelements 
of  great  sensitiveness  and  precision  which  give  the  rise  in  tempera- 
ture within  the  calorimeter  more  accurately  than  does  a  mercury 
thermometer;  electric  heating  and  vacuum  furnaces  for  the  pre- 
heating of  the  sample  without  contamination  to  the  desired 
high  temperature;  and  many  other  details  of  manipulation  and 
construction,  for  descriptions  of  which  the  reader  should  consult 
the  writings  of  Berthelot,  Louginine  arid  Schukarew,  Dickinson, 
Richards,  White,  Oberhoffer,  and  others. 

It  is  possible,  for  instance,  to  keep  the  total  error  due  to  the 
calorimeter  to  within  i  in  10,000.  These  improvements  are  of 
the  greatest  importance  for  the  exact  determination  of  specific 
and  latent  heats  and  similar  constants  at  high  temperatures,  but 
have  little  interest  from  the  purely  pyrometric  point  of  view, 
since  much  more  delicate  and  accurate  temperature-measuring 
methods  exist  which  do  not  involve  the  transfer  of  heat. 


100  HIGH  TEMPERATURES 

The  calorimetric  or  specific  heat  pyrometer  is  to  be  recom- 
mended for  certain  operations  below  1000°  C.  in  technical  works 
where  it  is  required  to  make  only  occasional  measurements  of 
moderate  precision;  in  cases  where  there  is  not  the  personnel 
sufficiently  skillful  to  use  the  more  precise  or  delicate  methods; 
and  finally,  where  the  importance  of  the  measurements  is  not 
such  as  to  justify  the  buying  of  more  costly  instruments. 


CHAPTER  IV. 
THERMOELECTRIC  PYROMETER. 

Principle.  —  The  junction  of  two  metals  heated  to  a  given 
temperature  is  the  seat  of  an  electromotive  force  which  is 
a  function  of  the  temperature  only,  at  least  under  certain 
conditions  which  we  shall  define  further  on.  In  a  circuit  in- 
cluding several  different  junctions  at  different  temperatures, 
the  total  electromotive  force  is  equal  to  their  algebraic  sum. 
In  a  closed  circuit  there  is  produced  a  current  equal  to  the 
quotient  of  this  resultant  electromotive  force  and  the  total 
resistance. 

Experiments  of  Becquerel,  Pouillet,  and  Regnault.  —  It  was 
Becquerel  who  first  had  the  idea  to  profit  from  the  discovery 
of  Seebeck  to  measure  high  temperatures  (1830).  He  used  a 
platinum-palladium  couple,  and  estimated  the  temperature  of 
the  flame  of  an  alcohol  lamp,  finding  it  equal  to  135°.  In  reality 
the  temperature  of  a  wire  heated  in  a  flame  is  not  that  of  the 
gases  in  combustion;  it  is  inferior  to  this. 

The  method  was  studied  and  used  for  the  first  time  in 
a  systematic  manner  by  Pouillet;  he  employed  an  iron- 
platinum  couple  which  he  compared  with  the  air  thermom- 
eter previously  described  (page  61).  In  order  to  protect  the 
platinum  from  the  action  of  the  furnace  gases,  he  inclosed  it 
in  an  iron  gun  barrel  which  constituted  the  second  metal  of 
the  junction.  Pouillet  does  not  seem  to  have  made  applica- 
tions of  this  method,  which  must  have  given  him  very  dis- 
cordant results. 

Edm.  Becquerel  resumed  the  study  of  his  father's  couple 
(platinum-palladium).  He  was  the  first  to  remark  the  great 
importance  of  using  in  these  measurements  a  galvanometer  of 
high  resistance.  It  is  the  electromotive  force  which  is  a  function 

101 


102  HIGH  TEMPERATURES 

of  the  temperature,  and  it  is  the  current  strength  that  is  meas- 
ured, Ohm's  law  gives 

E  =  RI. 

In  order  to  have  proportionality  between  these  quantities,  E  and 
7,  it  is  necessary  that  the  resistance  of  the  circuit  be  invariable. 
That  of  the  couple  necessarily  changes  when  it  is  heated;  this 
change  must  then  be  negligible  in  comparison  with  the  total 
resistance  of  the  circuit. 

Edm.  Becquerel  studied  the  platinum-palladium  couple  and 
made  use  of  it  as  intermediary  in  all  his  measurements  on  fusing 
points,  but  he  did  not  use  it,  properly  speaking,  as  a  pyrometer; 
he  compared  it,  at  the  instant  of  observation,  with  an  air  ther- 
mometer heated  to  a  temperature  near  to  that  which  he  wished 
to  measure.  He  also  tried  to  make  a  complete  calibration  of 
this  couple,  but  this  attempt  was  not  successful;  he  did  not  take 
into  account  the  irregularities  due  to  the  use  of  palladium; 
besides,  he  made  use  successively  for  this  graduation  of  a  mercury 
thermometer  and  of  an  air  thermometer  which  did  not  agree 
with  each  other.  He  was  led  to  assume  for  the  relation  between 
the  temperature  and  the  electromotive  force  a  very  complex 
expression;  the  formulae  which  he  gives  contain  together  twelve 
parameters,  while  with  the  parabolic  formula  of  Tait  and  Ave- 
narius  two  suffice;  thus 


which  well  represents  the  phenomenon  for  the  couple  in  question 
to  1500°. 

Regnault  took  up  the  study  of  Pouillet's  couple,  and  he  ob- 
served such  irregularities  that  he  condemned  unreservedly  the 
thermoelectric  method.  But  these  experiments  were  hardly  con- 
clusive, for  he  does  not  seem  to  have  considered  the  necessity  of 
using  a  high-resistance  galvanometer. 

Experiments  of  Le  Chatelier  and  of  Barus.  —  The  thermo- 
electric method  possesses,  nevertheless,  very  considerable  practi- 
cal advantages  for  use  in  the  laboratory  as  well  as  industrially, 
.such  as: 


THERMOELECTRIC   PYROMETER  103 

Smallness  of  thermoelectric  substance; 

Rapidity  of  indications; 

Possibility  of  placing  at  any  distance  the  measuring  apparatus. 

Le  Chatelier  decided  to  take  up  the  study  of  this  method, 
intending  at  the  outset  not  to  make  disappear  the  irregularities 
which  seemed  inherent  in  the  phenomena  in  question,  but  to 
study  the  law  of  these  irregularities,  so  as  to  determine  correc- 
tions which  would  permit  of  making  use  of  this  method,  at  least 
industrially,  for  approximate  measurements.  These  investiga- 
tions showed  in  their  turn  that  the  sources  of  error  observed 
could  be  suppressed;  the  principal  one,  and  the  only  serious  one, 
came  from  lack  of  homogeneity  of  the  metals  up  to  that  time 
employed. 

Barus,  whose  work  in  this  field  dates  from  1881,  studied  in 
great  detail  the  thermoelectric  measurement  of  high  tempera- 
tures as  well  as  the  advantages  and  limitations  of  the  various 
pyrometric  methods.  He  was  led  from  his  researches  to  prefer 
the  couple  Pt,  90  Pt-io  Ir. 

Iron,  nickel,  palladium,  and  their  alloys  were  found  to  be  un- 
suited  for  the  exact  measurement  of  high  temperatures,  because, 
heated  in  certain  of  their  points,  they  give  birth  to  parasite 
currents,  sometimes  relatively  intense.  D.  Berthelot  and  others, 
however,  have  since  used  successfully  in  oxidizing  atmospheres, 
thermocouples  with  palladium  as  one  element. 

As  an  example  of  inhomogeneity,  consider  the  electromotive 
forces  observed  by  Le  Chatelier  in  carrying  a  Bunsen  flame 
along  beneath  a  wire  of  ferronickel  of  i  mm.  diameter  and  50  cm. 
long;  the  electromotive  forces  are  expressed  in  microvolts  (mil- 
lionths  of  a  volt) : 

Distance 0.05      o.io      0.15        0.20      0.30      0.35    0.40      0.50 

E.M.F —200     +250     —150     —looo     —500     —200     —50     —200 

An  electromotive  force  of  1000  microvolts  is  that  given  by  the 
usual  couples  that  we  are  going  to  study  for  a  heating  of  100°. 
With  such  anomalies  as  above  there  could  hardly  be  any  meas- 
urements possible. 


104  HJGH  TEMPERATURES 

These  anomalies  may  sometimes  be  due  to  accidental  varia- 
tions in  the  composition  of  the  wires,  but  in  general  there  is  no 
preexisting  heterogeneity;  a  physical  heterogeneity  due  to  the 
heating  is  produced.  Iron  and  nickel,  heated  respectively  to 
750°  and  380°,  undergo  an  allotropic  transformation,  incompletely 
reversible  by  rapid  cooling. 

In  the  case  of  palladium,  there  may  be  produced,  in  a  reducing 
atmosphere,  phenomena  of  hydrogenation  which  change  com- 
pletely the  nature  of  the  metal,  so  that  a  metal  initially  homo- 
geneous may  become  by  simple  heating  quite  heterogeneous  and 
form  a  couple. 

Certain  metals  and  alloys  are  quite  free  from  these  faults, 
notably  platinum  and  its  alloys  with  iridium  and  rhodium.  The 
irregularities  previously  observed  are  thus  due  to  the  employ- 
ment of  iron  and  palladium  in  all  the  couples  tried. 

A  second  source  of  error,  less  important,  comes  from  the 
annealing.  In  heating  a  wire  at  the  dividing  point  between  the 
hardened  part  and  the  annealed  part,  there  is  developed  a  current 
whose  strength  varies  with  the  kind  of  wire  and  the  degree  of 
hardness.  The  twisting  that  a  wire  has  undergone  at  a  point 
suffices  to  produce  a  hardening.  A  couple  whose  wires  are  hard 
drawn  throughout  a  certain  length  will  give  different  indications 
according  to  the  point  of  the  wire  where  the  heating  ceases. 
Here  are  results  in  microvolts  obtained  by  Le  Chatelier  with  a 
platinum,  platinum-iridium  (20  per  cent  Ir)  couple  (platinum- 
indium  alloy  is  very  easily  annealed) : 

100°         445° 

Before  annealing noo     7200 

After  annealing 1300     7800 

Difference 200      600 

We  shall  now  study  successively: 

1.  The  choice  of  the  couple; 

2.  Thermoelectric  formulae; 

3.  The  methods  of  measurement; 

4.  The  sources  of  error; 

5.  The  standardization. 


THERMOELECTRIC   PYROMETER  105 

Choice  of  the  Couple.  —  We  shall  first  reproduce  the  evidence 
and  arguments  which  led  Le  Chatelier  to  prefer  and  introduce 
the  thermocouple  of  composition  platinum  against  its  alloy  with 
10  per  cent  rhodium  for  temperature  measurements  in  those 
cases  for  which  the  thermoelectric  method  is  preferable  or  con- 
venient. We  shall  then  give  account  of  some  of  the  later  work 
of  others  in  this  domain. 

In  the  choice  of  the  couple,  account  must  be  taken  of  the  elec- 
tromotive force,  the  absence  of  parasite  currents,  and  the  inalter- 
ability of  the  metals  used. 

Electromotive  Force.  —  This  varies  enormously  from  one  couple 
to  another.  Below  are  several  such  electromotive  forces  given 
between  o°  and  100°  by  metals  that  can  be  drawn  into  wires  and 
opposed  to  pure  platinum. 

Microvolts. 

Iron 2100 

Hard  steel 1800 

Silver : 900 

Cu  +  10%  Al 700 

Gold 600 

Pt  4- 10%  Rh  i 

Pt+io%Ir    I  S°° 

Cu  +  Ag 500 

Ferronickel 100 

Nickel  steel  (5%  Ni) o 

Manganese  steel  (13%  Mn) —   3°° 

Cu  +  20%  Ni -  600 

Cu  +  Fe  +  Ni -1200 

German  silver  (15%  Ni) -1200 

German  silver  (25%  Ni) -2200 

Nickel -2200 

Nickel  steel  (35%  Ni) -2700 

Nickel  steel  (75%  Ni) -3700 

Barus  studied  certain  alloys  between  o°  and  920°;  he  obtained 
the  following  results  against  platinum : 

Microvolts. 

Iridium  (2%) 79 1 

Iridium  (5%) 283° 

Iridium  (10%) S7oo 

Iridium  (15%) 7QOQ 

Iridium  (20%) 93OQ 

Palladium  (3%). 9»2 

Palladium  (10%) 93<x> 

Nickel  (2%) 3744 

Nickel  (5%) 


io6 


HIGH  TEMPERATURES 


Here  is  another  series  made  by  Barus  at  the  boiling  point  of 
sulphur  with  alloys  of  platinum  containing  2,  5,  and  10  per  cent 
of  another  metal: 


Metals. 

Au         Ag 

Pd 

Ir 

Cu 

2% 

s 

10 

2% 

5 

10 

2% 

5 
10 

-  242      -  18 
-  832      -105 
-1225      -158 

+  711 
+  869 
+  1127 

+  1384 
+  2035 
+3228 

+410 
+392 
+  257 

Ni 

Co 

Fe 

Cr 

Sn 

Zn 

+  2166 
+399° 
+  5095 

+  26 
-170 
—  41 

+3020 

+3313 
+3962 

+  2239 
+3123 
+3583 

+  261 
+  199 
+  151 

+396 
+  24 

Al 

Mn 

Mo 

Pb 

Sb 

Bi 

+  779 
+938 

+  758 
+  2206 

+  263 
+  1673 
+  766 

-268 
+338 

+  H55 

+  245 

Of  all  these  metals,  the  only  ones  to  keep  by  reason  of  their 
high  electromotive  force  are  the  alloys  of  platinum  with  iron, 
nickel,  chromium,  iridium,  and  rhodium.  The  following  table 
gives,  in  microvolts,  the  electromotive  forces  of  the  10  per  cent 
alloys  of  these  five  metals  up  to  the  temperature  of  1500°: 


Temperatures. 

Fe 

Ni 

Cr 

Ir 

Rh 

100° 

445 
920 

438 
3.962 
9,200 

646 
4,095 
9,100 

405 
3,583 

995 
6,39° 
14,670 

640 
3,690 
8,660 

1500 

19,900 

20,200 

26,010 

15,550 

Absence  of  Parasite  Currents.  —  The  alloy  with  nickel  gives 
parasite  currents  of  great  intensity,  as  do  all  the  alloys  of  this 
metal.  It  is  the  same  with  iron.  Chromium  does  not  seem  to 
present  the  same  inconvenience:  it  forms  an  alloy  difficult  to 
fuse  and,  for  this  reason,  difficult  to  prepare.  With  the  alloys 


THERMOELECTRIC  PYROMETER  107 

of  indium  and  of  rhodium  there  is  no  considerable  production 
of  parasite  currents  if  the  metals  are  pure  and  the  alloys  homo- 
geneous. 

There  remain,  then,  but  three  metals  to  consider:  iridium, 
rhodium,  and  chromium.  Of  the  alloys  of  these  metals  with 
platinum,  that  of  iridium  is  the  one  which  hardens  the  most 
easily. 

Chemical  Changes.  —  All  the  alloys  of  platinum  are  slightly 
alterable.  Those  of  nickel  and  of  iron,  at  high  temperatures, 
assume  a  slight  superficial  brownish  tint  caused  by  oxidation 
of  the  metal.  No  test  has  been  made  to  see  if,  after  a  long 
time,  this  attack  would  reach  even  to  the  interior  of  the 
wires. 

According  to  Le  Chatelier  the  alloys  of  platinum,  and  plati- 
num itself,  become  brittle  by  simply  heating  them  long  enough, 
especially  between  1000°  and  1200°;  this  is  due  without  doubt 
to  crystallization.  The  platinum-iridium  alloy  undergoes  this 
change  much  more  rapidly  than  the  platinum-rhodium,  and  this 
latter  more  rapidly  than  pure  platinum.  It  is  questionable, 
however,  if  this  effect,  other  than  a  slight  crystallization,  occurs 
in  a  strictly  oxidizing  atmosphere  with  couples  containing  only 
Pt,  Rh,  or  Ir. 

But  a  much  more  grave  cause  of  the  alteration  of  platinum 
and  its  alloys  is  the  heating  to  high  temperatures  in  a  reducing 
atmosphere. 

All  the  volatile  metals  attack  platinum  very  rapidly,  and  a 
great  number  of  metals  are  volatile.  Copper,  zinc,  silver,  anti- 
mony, nickel,  cobalt,  and  palladium,  at  their  points  of  fusion, 
already  emit  a  sufficient  quantity  of  vapor  to  alter  rapidly  the 
platinum  wires  placed  in  the  neighborhood.  These  metallic 
vapors,  that  of  silver  and  palladium  excepted,  can  only  exist  in 
a  reducing  atmosphere.  Among  the  metalloids,  the  vapors  of 
phosphorus  and  of  certain  compounds  of  silicon  are  particularly 
dangerous.  It  is  true  that  one  is  rarely  concerned  with  these 
uncombined  true  metalloids,  but  their  oxides  in  the  presence  of 
a  reducing  atmosphere  are  more  or  less  completely  reduced.  In 


108  HIGH  TEMPERATURES 

the  case  of  phosphorus,  it  is  not  only  necessary  to  shun  phosphoric 
acid,  but  also  acid  phosphates  of  all  the  metals  and  the  basic 
phosphates  of  the  reducible  oxides;  thus  silicon,  silica,  and  almost 
all  the  silicates,  clay  included,  must  be  avoided  if  a  reducing 
atmosphere  is  employed. 

The  reducing  flames  in  a  fire-clay  furnace  lead  little  by  little 
to  the  destruction  of  the  platinum  wires.  It  is  thus  indispen- 
sable to  protect  the  couples  against  any  reducing  atmosphere  by 
methods  which  will  be  indicated  further  on. 

In  taking  account  of  these  different  considerations,  —  electro- 
motive force,  homogeneity,  hardness,  alterability  by  fire,  —  Le 
Chatelier  was  led  to  give  the  preference  to  the  couple  Pt  —  Pt  + 
10%  Rh,  with  the  possibility  of  replacing  the  rhodium  by  irid- 
ium  and  perhaps  by  chromium.  In  all  cases  the  wires  should  be 
annealed  electrically  to  1400°  before  using. 

The  usual  diameter  of  wire  employed  is  0.6  mm.,  but  one  of 
0.4  mm.  contains  only  half  as  much  metal,  and  even  for  most  in- 
dustrial purposes  is  of  sufficient  robustness.  In  the  laboratory 
there  is  advantage,  especially  on  account  of  heat  conduction,  to 
still  further  reduce  this  diameter. 

Thermoelectric  Formulae.  —  In  spite  of  numerous  attempts 
to  solve  the  problem,  it  has  thus  far  been  impossible  to  deduce 
from  purely  theoretical  grounds  a  satisfactory  equation  connect- 
ing the  temperature  and  electromotive  force  of  any  thermo- 
electric couple.  As  we  shall  see,  it  is  necessary  to  set  up,  for 
each  type  of  couple,  an  empirical  equation  or  a  series  of  such 
equations  which,  sometimes  within  rather  restricted  temperature 
limits,  represents  well  enough  the  desired  relation.  There  is  a 
great  diversity  of  such  formulae,  and  there  has  been  in  the  past  a 
considerable  amount  of  indiscriminate  and  unwarranted  extrap- 
olation of  such  empirical  relations  to  temperature  regions  both 
high  and  low,  in  which  the  assumed  formulae  do  not  hold.  From 
the  very  common  use  of  the  thermocouple  as  a  temperature- 
indicating  device,  this  practice  has  caused  considerable  confusion 
in  the  values  to  be  assigned  to  high  temperatures.  We  shall 
call  attention  to  some  of  the  formulae  that  have  been  used  and 


THERMOELECTRIC  PYROMETER 


I09 


point  out  their  limitations  both  for  interpolation  and  extrapo- 
lation. 

In  the  construction  of  thermoelectric  formulae  it  is  customary 
to  assume  a  constant  temperature,  usually  o°  C.,  for  the  cold 
junctions,  and  to  further  assume  that  the  only  source  of  E.M.F. 
is  the  hot  junction.  The  complete  expression,  however,  for  the 
total  E.M.F.  developed  in  a  thermoelectric  circuit  requires  ac- 
count to  be  taken  also  of  (i)  the  Thomson  effect,  or  the  E.M.F.'s 
.generated  due  to  differences  in  temperature  along  a  homogeneous 
wire;  (2)  the  Peltier  effect  due  to  the  heating  of  the  junction  of 
two  dissimilar  metals  anywhere  in  the  circuit;  (3)  the  Becquerel 
effect,  or  the  E.M.F.'s  developed  by  physical  or  chemical  in- 
homogeneity  in  a  single  wire. 

The  E.M.F.  actually  measured  is  the  algebraic  sum  of  all  these 
quantities.  Practically,  the  Thomson  effect  need  not  be  taken 
account  of  separately  in  constructing  a  formula,  as  it  is  a  function 
only  of  the  temperature  difference  along  the  wires  and  of  their 
nature. 

The  undesirable  Peltier  and  Becquerel  effects,  the  former  oc- 
curring often  in  the  measuring  apparatus,  and  the  latter  mainly 
in  the  thermocouple  wires,  cannot  be  taken  care  of  numerically 
in  any  useful  thermoelectric  formula,  and  must  therefore  be 
eliminated  by  the  use  of  materials  and  methods  free  from  these 
effects. 

The  following  formulae,  therefore,  all  assume  thermoelectric 
circuits  in  which  the  only  sources  of  E.M.F.  are  due  to  the  dif- 
ference in  temperature  between  the  hot  and  cold  junctions  of  the 
couple. 

Thermoelectric  Power.  —  By  differentiating  with  respect  to 
temperature  the  expression  for  the  E.M.F.-temperature  relation 

E  =f(f)  for  any  couple,  we  get  a  quantity  known  as  its  thermo- 

j  -p 
electric  power,  — ,  which  we  may  designate  by  H.    This  quantity 

is  a  convenient  one  with  which  to  compare  the  numerical  be- 
havior at  any  temperature  of  two  or  more  couples,  or  of  one 
couple  at  different  temperatures,  as  it  gives  the  E.M.F.  per  de- 


no 


HIGH  TEMPERATURES 


gree  of  temperature.  For  some  couples  H  is  practically  a  linear 
function  of  t  over  considerable  ranges  of  temperatures,  i.e., 
H  =  a  -f  bt  and  is  a  measure  of  the  sensibility  of  any  type  of 
couple.  We  may  cite  the  following  as  illustrations: 

THERMOELECTRIC  POWERS  OF  THERMOCOUPLES. 


_,,                  ,                 Thermoelectric  Temperature 

power  (microvolts).  range. 

Pt,  90  Pt—  10  Rh    4. 3  +o. 0088  t  0-1300 

Pt,  QoPt— loir    11.3 -j- 0.0104  /  o-iooo 

Pt,  Ni                         7. 8  +  0. 01325  £  300-1300 

Cu,  Ni                      24.4  +  0.016  /  0-235 

Cu,  Constantan      42 . 3  +  o .  058  t  0-320 

Pt  —  Fe  (forged)      2 . 5  +  o .  02 10  t  700-1000 


Author. 

Le  Chatelier 

Le  Chatelier 

Burgess 

Pecheux 

Pecheux 

Le  Chatelier 


It  is  usual  to  express  H  for  a  single  substance  in  terms  of  lead 
as  a  standard  at  ordinary  temperatures,  but  at  high  tempera- 
tures this  becomes  impracticable.  The  values  of  H  for  steels 
are  of  special  interest  in  view  of  their  use  in  many  base-metal 


dE 


18 


10 


loo  °c 


- 


500 


1000° 


Fig.  23.     Thermoelectricity  of  Steels. 


couples.  In  Fig.  23,  due  to  Belloc,  are  given  the  changes  of  H,. 
with  temperature  and  carbon  content  for  various  steels  against 
Pt,  from  which  it  is  evident  that  thermocouples  with  steel  or  iron 
as  one  component  have  complex  E.M.F.-temperature  relations, 
and  that  the  relation  between  thermoelectric  power  and  tempera- 
ture is  far  from  linear. 


THERMOELECTRIC   PYROMETER  III 

For  some  couples  the  thermoelectric  powers  of  the  component 
wires  become  equal  and  opposite  in  sign  at  some  temperature 
known  as  the  neutral  point,  beyond  which  the  sign  of  the  E.M.F. 
is  negative.  It  is  evidently  of  advantage  to  use  couples  in 
regions  removed  from  their  neutral  point. 

As  shown  by  Stansfield,  the  Peltier  effect  I  T~  jis  very  nearly 

linear  with  the  temperature  for  the  Pt-Rh  and  Pt-Ir  couples, 
but  not  the  thermoelectric  power.  Sosman's  observations  on 
various  Pt-Rh  couples  also  bear  out  this  statement. 

Formula.  —  Avenarius  and  Tait  have  shown  that  up  to  300° 
the  electromotive  force  of  a  great  number  of  couples  is  repre- 
sented in  a  manner  sufficiently  exact  by  means  of  a  parabolic 
formula  of  two  terms: 


The  experiments  of  Le  Chatelier  on  the  platinum-palladium 
couple  have  shown  that  the  same  formula  holds  also  for  this 
couple  up  to  the  fusing  point  of  palladium: 


e  =  4-3  *  - 

IOOO 

/  =  loo  445  954  1,060  i>55° 

e  =  500  2950  10,900  12,260  24,030 

Platinum  and  Its  Alloys.  —  This  law  fails  completely,  however, 
for  couples  made  of  pure  platinum  and  an  alloy  of  this  metal. 

Here  are  three  early  series  of  determinations  made  with  dif- 
ferent couples,  giving  an  idea  of  the  order  of  magnitude  of  the 
E.M.F.'s  of  thermocouples  of  types  used  very  frequently,  as 
determined  by  these  observers. 


Barus. 
Pt  -  Pt  10%  Ir. 

Le  Chatelier. 
Pt  -  Pt  10%  Rh. 

Holborn  and  Wien. 
Pt  -  Pt  10%  Rh. 

I 

e 

t 

e 

J 

e 

300 

2,800 

IOO 

550 

IOO 

565 

500 

5.250 

357 

2,770 

2OO 

1,260 

700 

7,900 

445 

3.630 

4OO 

3.030 

900 

10,050 

665 

6,180 

600 

4,920 

I  IOO 

13,800 

1060 

10,560 

800 

6,970 

155° 

16,100 

IOOO 

9,080 

1780 

18,200 

I2OO 

11,460 

I4OO 

13,860 

l6oO 

16,220 

112  HIGH  TEMPERATURES 

Holman  showed  that  the  results  of  Holborn  and  Wien  may  be 
expressed  by  a  logarithmic  formula  containing  only  two  param- 
eters and  requiring,  therefore,  only  two  calibration  temperatures. 
Le  Chatelier  showed  likewise  that  his  results  could  also  be  repre- 
sented by  the  Holman  formula,  and  in  general  it  may  be  said 
that  for  use  below  1200°  C.  of  the  thermocouple  made  of  platinum 
and  its  alloys  with  rhodium  and  iridium,  the  logarithmic  formula 
satisfies  the  results  of  observations  to  2°  C.,  or  well  within  the 
limits  of  all  except  the  most  accurate  work. 

Holman's  formula  is  as  follows: 

(i) 

x_^  < 

where  ^  e  is  the  electromotive  force  of  the  couple  for  any  tem- 
o 

perature  /  when  the  cold  junction  is  kept  at  zero  centigrade. 
The  two  constants  are  readily  computed  or  evaluated  graphically, 
and  the  resultirig  plot  serves  indefinitely  for  the  determination 
of  any  temperature  with  a  given  couple.  The  equation  does  not 
apply  in  the  region  in  which  the  thermocouple  is  insensitive,  that 
is,  below  250°  C.  It  may  be  written,  for  convenience  in  plotting 
and  computation: 


(2)  log  2/  e  =n\Qgt  +  log  m; 


so  that  if  log  e  be  plotted  as  abscissas  and  log  /  as  ordinates,  a 
straight  line  is  obtained. 

This  formula  has  been  applied  successfully  to  the  above  obser- 
vations of  L^  Chatelier  on  platinum-rhodium  couples  and  to  those 
of  Barus  on  platinum-iridium  couples. 

Holborn  and  Day,  in  their  very  elaborate,  direct  comparison 
of  the  nitrogen  thermometer  with  thermocouples  made  of  the 
various  platinum  metals,  in  the  interval  300°  to  1100°  C.,  found 
that  if  a  precision  of  i°  is  sought,  a  three- term  formula  is  required 
to  express  the  relation  between  E.M.F.  and  temperature. 

The  formula 

(3) 


THERMOELECTRIC  PYROMETER 

is  the  one  they  have  used.  The  labor  involved  in  computation 
with  this  form  is  considerable,  and,  unless  a  very  great  accuracy 
is  required,  Holman's  formula  is  amply  sufficient,  when  the 
uncertainty  of  the  absolute  values  of  high  temperatures  is  con- 
sidered. 

Stansfield  deduces  from  theoretical  considerations  the  formula 

\4/  ffT  •      J 

which  may  be  written 

a  form  which  satisfies  the  experimental  results  determined  with 
pure  platinum  wires.  This  form  possesses  no  practical  advan- 
tage over  that  of  Holborn  and  Day,  unless  it  be  its  usefulness, 
by  employing  the  graphical  method,  in  detecting  slight  errors  in 

fusing  points.     The  values  of  —  at  the  points  of  fusion  can  be 

de 

obtained  from  the  T  vs.  e  plot,  and  the  T  vs.  —  curve  thus  con- 
structed throws  into  prominence  the  experimental  errors  at  these 
points.  As  the  above  formulae  indicate,  the  curve  for  the  plati- 
num metals  constructed  with  T  as  abscissas  and  T  •  -  •  as 

al 

ordinates  is  a  straight  line.  The  errors  of  the  method  are  less 
than  2°  at  1000°.  The  ordinary  metals,  on  the  other  hand,  with 
a  few  exceptions  such  as  nickel  and  cobalt,  give  nearly  a  straight 

de 

line  for  the  curve  T  vs.  —  • 
al 

A  formula  which  has  been  used  on  account  of  its  more  con- 
venient form,  than  (3)  for  example,  in  the  computation  of  tem- 
perature, is: 

(6)  /  =  a  +  be  -  ce2. 

This  formula  satisfies  the  observations  with  platinum-rhodium 
and  platinum-iridium  couples  in  the  range  300°  to  1100°  C.  almost 
as  well  as  (3). 


114 


HIGH  TEMPERATURES 


We  may  compare  these  various  formulae  by  computing  their 
deviations  at  various  fixed  points,  making  use  of  the  latest  data 
with  comparison  of  a  thermocouple  (90  Pt  - 10  Rh-Pt)  with  the 
gas-thermometer  scale,  —  those  of  Day  and  Sosman,  1910.  We 
shall  assume  as  calibration  temperatures  for  the  three-term 
equations,  (3),  (5),  and  (6),  the  freezing  points  of  zinc,  antimony, 
and  copper,  and  for  Holman's  equation  (2),  zinc  and  copper. 

COMPARISON    OF  THERMOELECTRIC  FORMULA 
(Pt-QO  Pt  -ioRh) 


Substance. 

Freezing 
point. 

Observed 
(microvolts). 

Observed  —  calculated  temperatures. 
(2)'                  (3)                    (5)                 (6) 

Cadmium 

320.0° 

2,502 

—  O.2 

-0.3 

+  6.9 

—   I.I 

Zinc 

418.2 

3.429 

0 

0 

o 

0 

Antimony 

629.2 

5.529 

+  2.3 

0 

o 

—  o.i 

Silver 

960.0 

9,111 

+2.5 

+    0.2 

+    2.2 

-  0.9 

Gold 

1062.4 

10,296 

+0.4 

+    0.2 

O 

0 

Copper 

1082.6 

io.535 

o 

O 

O 

+    O.I 

Diopside 

1391 

14,231 

-6 

+  10 

—  10 

+19 

Nickel 

I452 

14,969 

-6 

+  14 

—  II 

+28 

Cobalt 

1490 

15,423 

-7 

+  14 

-13 

+3i 

Palladium 

J549 

16,140 

-5 

+  20 

—  14 

+42 

Platinum 

1755 

18,613 

+  i 

+42 

-15 

+  73 

The  numbers  in  parentheses  refer  to  formulae  on  preceding  pages. 


It  is  evident  from  the  table  that  we  have,  therefore,  as  many 
thermoelectric  scales  as  we  have  equations.  The  two  formulae 
which  best  fit  the  region  300°  to  1100°  C.,  namely,  (3)  and  (6),  are 
clearly  not  suited  for  extrapolation  without  applying  proper 
corrections.  Of  all  the  formulae,  Holman's  (2),  which  is  also  the 
simplest,  is  the  best  suited  for  general  use  throughout  the  whole 
range  300°  to  1750°,  giving  a  maximum  error  of  2.5°  below,  and 
of  7°  above,  1100°  C.  None  of  these  equations  is  satisfactory  for 
the  most  exact  work,  however.  A  cubic  equation  in  /  will  satisfy 
the  data  more  exactly,  but  this  is  extremely  inconvenient  to 
solve  for  t\  or  two  parabolas  of  type  (3)  may  be  used,  the  first  from 
300°  to  1100°,  the  second  from  1100°  to  1750°. 

In  1905,  Harker,  using  thermocouples  of  platinum  against  a 
10  per  cent  rhodium  and  10  per  cent  iridium  alloy  of  plati- 
num, respectively,  and  extrapolating  equation  (3)  from  uoo°C., 


THERMOELECTRIC   PYROMETER  115 

obtained  1710°  C.  with  both  types  of  thermocouple  as  the  uncor- 
rected  value  of  the  platinum  melting  point.  This  value,  1  7  10°  C  ., 
has  been  generally  accepted  in  many  quarters  as  the  true  melting 
point  of  this  metal.  Waidner  and  Burgess,  however,  demon- 
strated in  1907  that  the  value  found  for  high  melting  points  by 
extrapolating  with  thermocouples  depends  not  only  on  the  ther- 
moelectric relation  assumed,  but  also  on  the  nature  of  the  couple. 
Some  of  their  results  for  the  palladium  and  platinum  melting 
points  are  given  below,  the  calibration  equations  and  tempera- 
tures being  the  same  as  in  the  above. 

EXTRAPOLATION   WITH   VARIOUS  THERMOCOUPLES. 

Tvnp  nf  rnimlp  Equa-  Palladium,  Platinum, 

tion.  MP  =  1549-  MP  =  I75S. 

4of  Pt,9oPt  -  ioRh(approx.)      ((3)  1521°  to  1537°  1698°  to  1715° 

2  makers  ...................  }  (2)  1536    to  1561  1717    to  1754 

2  of  Pt,  90  Pt  —  10  Ir  (  (3)  1525    to  1528  1705    to  1710 

2  makers  ..................  1(2)  1516    to  1541  1697    to  1728 

2of9oPt-IORh,8oPt-2oRhj(3)         J507  ,687    to  ,7,0 


It  would  appear  from  these  data  that  the  corrections  to  apply 
to  a  given  type  of  thermocouple  computed  and  extrapolated  with 
a  given  formula  are  uncertain,  the  slight  variations  in  composi- 
tion of  the  alloy  wire  from  one  couple  to  another  apparently 
producing  considerable  differences  in  the  computed  temperatures. 

It  is  an  interesting  fact  that  the  10  per  cent  alloys  of  Rh  and  Ir 
with  Pt,  when  treated  by  equation  (3),  give  very  exactly  the  same 
temperature  scale  to  the  melting  point  of  platinum,  although  the 
actual  shapes  of  the  E.M.F.  temperature  curves  are  very  different 
for  those  two  couples,  that  for  Pt  —  Ir  being  the  more  nearly 
linear.  It  was  an  instructive  case  of  two  negatives  not  making 
an  affirmative  to  assign  the  value  1710°  as  the  true  Pt  melting 
point  because  both  Ir  and  Rh  couples  led  to  the  same  result. 

Using  Pt-Rh  couples  of  i,  5,  10,  and  15  per  cent  Rh,  and  cali- 
brating in  terms  of  equation  (3)  at  the  melting  points  of  copper, 
diopside,  and  palladium  (see  above),  Sosman  finds  1752°  as  a 
mean  value  for  Pt  with  a  range  of  only  7°. 


Il6  HIGH  TEMPERATURES 

Variation  of  E.M.F.  with  Composition.  —  Sosman  has  also 
studied  this  for  the  Pt-Rh  couples,  and  some  of  his  results  are 
given  in  Fig.  24.  It  will  be  noticed  that  in  the  region  of  the  10 
per  cent  alloy,  which  is  the  one  most  commonly  met  with,  or  at 
least  such  is  the  nominal  composition  usually  given,  a  change 
of  i  per  cent  in  composition  is  equivalent  to  about  50°  at 
1000°. 

The  Base-metal  Couples.  —  The  E.M.F.-temperature  relation 
for  some  of  these  couples,  of  which  there  are  a  great  many  in  use, 
is  very  nearly  linear.  For  some  couples,  on  the  other  hand,  the 
E.M.F.-temperature  relation  is  very  complex;  and  in  those  cases 
in  which  there  are  allotropic  or  other  transformations  within 
the  material,  taking  place  over  a  temperature  range  or  along  the 
wire  as  the  successive  portions  are  heated  or  cooled,  there  some- 
times occur  inflections  in  the  curve,  producing  regions  of  con- 
siderable extent  in  which  the  couple  is  relatively  very  insensitive. 
When  such  inflections  occur,  there  is  usually  no  conveniently 
expressed  relation  between  E.M.F.  and  temperature  (see  Fig.  23). 
We  shall  call  attention  later  to  some  specific  cases  of  base-metal 
thermoelectric  formulae. 

Methods  of  Measurement  of  Temperature.  —  Two  methods 
may  be  used  to  measure  the  electromotive  force  of  a  couple:  the 
method  of  opposition  and  the  galvanometric  method.  From  the 
scientific  point  of  view,  the  first  alone  is  rigorous;  it  is  usually 
made  use  of  in  laboratories.  The  second  method  is  simpler,  but 
possesses  the  inconvenience  of  giving  only  indirectly  the  measure 
of  the  electromotive  force  by  means  of  a  measurement  of  current 
strength.  This  inconvenience  is  more  apparent  than  real  in  the 
later  forms  of  instrument,  as  will  be  shown. 

There  are  sources  of  error,  however,  inherent  in  the  galvano- 
metric method,  such  as  effects  of  lead  resistance  and  temperature 
coefficients  of  leads  and  galvanometer,  which,  as  we  shall  see, 
are  difficult  if  not  impossible  of  complete  elimination  even  with 
the  best  apparatus  available.  The  method  of  opposition,  on 
the  other  hand,  may  be  made,  in  so  far  as  the  measurements 
of  E.M.F.  are  concerned,  as  exact  as  may  be  desired,  or  so  that 


THERMOELECTRIC  PYROMETER 


117 


/ll 


\l 


J 


f 


A 


100°  200°  300°  400°  500°  600°  700°  800°  900°  1000°  1100°  1200°  1300°  1400°  1500°  1600°  1700 1755° 

Temperature,  Centigrade 
Fig.  24.     E.M.F.  of  Pt-Rh  Thermocouples. 


Il8  HIGH   TEMPERATURES 

the  only  outstanding  uncertainties  are  inherent  in  the  thermo- 
couple itself.  These  uncertainties,  such  as  inhomogeneity  and 
conduction  along  the  wires,  variable  zero,  and  actual  change  of 
E.M.F.,  are  sometimes  overlooked,  giving  rise  to  illusory  accu- 
racy. 

We  shall  describe  each  of  these  methods  and  discuss  their 

limitations,  and  also  point  out  the  sources  of  error  most  likely  to 

\  be  present  with  the  various  types  of  thermoelectric  apparatus. 

Galva^on^etric  Method.  —  The  measurement  of  an  electro- 
motive force  may  be  reduced  to  that  of  a  current;  it  suffices  for 
that  to  put  the  couple  in  a  circuit  of  known  resistance,  and  from 
Ohm's  law  we  have 

*-£:"• 

If  the  resistance  is  not  known,  but  is  constant,  the  electro- 
motive force  will  be  proportional  to  the  current  strength,  and 
that  will  suffice,  on  the  condition  that  the  calibration  of  the 
couple  is  made  with  the  same  resistance.  If  this  resistance  is 
only  approximately  constant,  the  relation  of  proportionality  will 
be  only  approximately  exact. 

This  method  is  the  one  used  in  practically  all  industrial  prac- 
tice, and  to-day  galvanometers  can  be  had  satisfying  all  the  re- 
quirements of  which  we  shall  treat  in  the  following  paragraphs. 
In  many  quarters  the  thermoelectric  pyrometer  has  been  dis- 
credited because  instruments  giving  evidently  unreliable  results 
were  used.  With  a  better  understanding  of  the  requirements 
and  the  meeting  of  them  by  manufacturers,  this  prejudice  is 
•  disappearing. 

\  Resistance  of  Couples  and  Galvanometer.  —  The  wires  of  the 
Wuple  make  necessarily  a  part  of  the  circuit  in  which  the 
Current  strength  is  measured,  and  their  resistance  varies  with 
increase  of  temperature.  It  is  important  to  take  account  of 
the  order  of  magnitude  of  this  inevitable  change  of  resist- 
ance. 

Barus  made  a  systematic  series  of  observations  on  the  alloys 
of  platinum  with  10  per  cent  of  other  metal.  The  relation 


THERMOELECTRIC  PYROMETER  119 

between  the  resistance  and  the  temperature  being  of  the  form 

Rt  =  Ro  (i  +  at), 
he  obtained  the  following  results: 


Pt 

(pure) 

Au 

Ag" 

Pd 

Ir 

Cu 

Ni 

Fe 

Cr 

Sn 

Specific  resistance  in  mi- 

crohms (R)  

1C.  * 

75  6 

34.8 

23  o 

24  4 

63  o 

•?•?    7 

N6 

42 

3O 

1000  a  

2.2 

I 

0.7 

1.2 

1.2 

O.2 

O.Q 

0.4 

0-5 

0.7 

Other  tests  gave  the  figures  below: 


5% 

Al 

& 

10% 
Mo 

spf 

2% 

Sb 

$ 

% 

5% 
Zn 

Ro  

lOOOtt 

22 
I    t; 

So 
o  4 

I7.6 
I  O 

7-7 
i  8 

29.5 

I 

16.6 

2 

47.8 

O  3 

25 
j    i 

The  coefficient  a  is  taken  between  o°  and  357°  (boiling  point 
of  mercury). 

The  experiments  of  Le  Chatelier,  for  the  couples  that  he  used, 
gave  the  following  results: 

For  platinum, 

R  =  ii. 2  (i  +  0.002  i)  between  o°  and  1000°. 
For  platinum-rhodium  (10%  Rh), 

R  =  27  (i  +  0.0013  /)  between  o°  and  1000°. 
Holborn  and  Wien  found  for  pure  platinum, 

R  =  7.9  (i  +  0.0031 1)  between  o°  and  100°, 
R  =  7.9  (i  +  0.0028  /)  between  o°  and  1000°. 

Very  commonly  couples  are  made  of  the  platinum  metals  of 
wires  i  m.  in  length  and  0.5  mm.  in  diameter;  their  resistance, 
which  is  about  2  ohms  cold,  is  doubled  at  1000°.  If  use  is  made 
then  of  a  galvanometer  of  a  resistance  of  200  ohms,  and  if  the 
variation  of  the  resistance  of  the  couple  is  neglected,  the  error  is 
equal  to  T^¥.  In  general  this  error  is  still  less  except  in  certain 


120  HIGH  TEMPERATURES 

industrial  uses.    Thus  in  the  laboratory  the  length  heated  is  often 
less  than  10  cm.,  and  then  the  error  reduces  to  y^Vo- 

We  may  calculate  the  effect  of  resistance  in  the  electrical  cir- 
cuit, including  that  of  the  couple  and  galvanometer,  on  the  read- 
ing of  the  pyrometer  galvanometer  in  the  following  way:  If  E 
is  the  true  E.M.F.  generated  by  the  thermocouple  and  E'  the 
E.M.F.  indicated  by  a  galvanometer  of  resistance  R,  in  series 
with  the  couple  and  leads,  of  resistance  r  and  r'  respectively, 

then 


In  the  case  of  certain  industrial  installations,  where  the  galva- 
nometer is  at  a  distance  from  the  couple,  the  value  of  r'  ,  the 
resistance  of  the  copper  wires  connecting  the  couple  to  the  gal- 
vanometer, may  be  of  as  great  importance  as  that  of  the  couple 
wires,  r.  The  value  of  v'  can  of  course  be  kept  down,  however, 
by  increasing  the  size  of  wire  used. 

Although  in  the  case  of  platinum  couples,  which  on  account  of 
cost,  high  specific  resistance,  and  temperature  coefficient  of  the 
materials  necessarily  have  an  appreciable  resistance  and  therefore 
require  a  relatively  high  resistance  galvanometer,  it  should  be 
noted  that,  with  base-metal  couples  of  large  cross  section  and 
consequently  low  resistance,  galvanometers  of  very  much  lower 
resistance,  and  therefore  of  a  more  robust  type,  in  general,  may  be 
allowed  here.  For  example,  if  the  couple  has  a  resistance  of 
o.i  ohm  and  the  connecting  leads  a  negligible  resistance,  as  may 
readily  happen  with  certain  types  of  pyrometer  rod,  the  gal- 
vanometer may  be  a  millivoltmeter  of  only  10  ohms  without 
introducing  errors  over  T^,  or  10°  at  1000°  C.,  due  to  this 
cause. 

Pyrometer  Galvanometers.  —  It  may  still  be  of  interest  to 
recall  the  historical  development  of  this  phase  of  the  subject,  as 
it  offers  a  good  illustration  of  the  influence  of  one  field  of  activity 
on  another,  and  from  the  fact  that  the  difficulties  encountered 
and  the  precautions  to  be  taken  in  the  construction  and  use  of 
these  instruments  are  not  yet  sufficiently  well  appreciated  by 


THERMOELECTRIC   PYROMETER 


121 


some  manufacturers  as  well  as  by  many  experimenters  and  other 
users. 

The  earliest  measurements,  those  of  Becquerel  and  of  Pouillet, 
were  made  with  needle  galvanometers  controlled  by  terrestrial 
magnetism.  Such  apparatus,  sensible  to  jarring,  requires  deli- 
cate adjustment,  and  the  readings  take  a  long  time.  The  use 
of  these  instruments  would  have  prevented  the  method  from  be- 
coming practical.  It  is  only  thanks  to  the  use  of  movable-coil 
galvanometers  of  the  Deprez-d'Arsonval  type  that  the  thermo- 
electric pyrometer  has  been  able  to  become,  as  it  is  to-day,  an 
apparatus  in  current  usage. 

This  apparatus,  in  one  of  its  earlier  forms  (Fig.  25),  is  composed 
of  a  large  horseshoe  magnet  between  whose  poles  is  suspended  a 
movable  frame  through  which  the  cur- 
rent passes.  The  metallic  wires,  which 
serve  at  the  same  time  to  suspend  the 
coil  and  bring  in  the  current,  undergo 
then  a  torsion  which  is  opposed  to  the 
displacement  of  the  coil. 

The  latter  stops  in  a  position  of 
equilibrium  which  depends  both  on  the 
strength  of  the  current  and  the  value 
of  the  torsion  couple  of  the  wires. 
To  these  two  forces  is  added,  in  gen- 
eral, a  third,  due  to  the  weight  of  the 
coil,  which  causes  disturbing  effects 
often  very  troublesome.  We  shall  speak  of  this  further  on. 

The  measurement  of  the  angular  displacement  of  the  coil  is 
made  sometimes  by  means  of  a  pointer  which  swings  over  a 
divided  scale,  more  often  by  means  of  a  mirror  which  reflects  on 
a  semitransparent  scale  the  image  of  a  wire  stretched  before  a 
small  opening  conveniently  lighted. 

These  movable-coil  galvanometers  were  for  a  long  time  con- 
sidered by  physicists  as  unsuited  for  any  quantitative  measure- 
ments; they  were  only  employed  in  null  methods  and  made 
accordingly.  In  order  to  render  them  suitable  for  quantitative 


Fig.  25.     Moving-coil 
Galvanometer. 


122  HIGH  TEMPERATURES 

measurements  of  current,  it  was  necessary  to  attend  to  a  series  of 
details  of  construction,  previously  neglected.  Here  are  the  most 
important  among  these,  as  noted  by  Le  Chatelier  for  suspended- 
coil  galvanometers: 

1.  The  movable  coil  should  possess  a  resistance  as  little  variable 
as  possible  with  the  surrounding  temperature  in  order  to  avoid 
corrections  always  very  uncertain.     The  coils  of  copper  wire 
ordinarily  used  to  augment  the  sensibility  should  be  absolutely 
discarded;  use  should  be  made  of  coils  of  German  silver  or  of  sim- 
ilar metal  with  small  temperature  coefficient  such  as  manganin. 

2.  The  spaces  which  separate  the  coils,  from  the  poles  of  the 
magnet,  on  the  one  hand,  and  from  the  central  soft-iron  core 
on  the  other,  should  be  sufficiently  great  to  avoid  with  certainty 
any  accidental  friction  which  would  prevent  the  free  movement 
of  the  coil.     The  rubbings  to  look  out  for  do  not  come  from  the 
direct  contact  of  the  frame  with  the  magnet:  these  latter  are  too 
visible  to  escape  unseen.    Those  which  are  to  be  guarded  against 
come  from  the  rubbing  of  filaments  of  silk  which  stand  out  from 
insulating  covering  of  the  metallic  wires,  and  from  the  ferruginous 
dust  which  clings  to  the  magnet.     It  is  here,  it  would  seem,  that 
the  most  serious  source  of  error  is  met  with  in  the  use  of  the 
movable-coil  galvanometer  as  measuring  instrument.    There  is 
no  warning  indication  of  these  slight  rubbings  which  limit  the 
displacement  of  the  coil  without,  however,  taking  from  it  its 
apparent  mobility. 

3.  The  suspending  wire  should  be  as  strong  as  may  be  to 
support  the  coil  without  being  exposed  to  breaking  by  shocks; 
on  the  other  hand,  it  should  be  very  fine,  so  as  not  to  have  too 
great  a  torsion  couple.     Two  different  artifices  help  to  reconcile 
somewhat  these  two  opposed  conditions:  the  use  of  the  mode  of 
suspension  of  Ayrton  and  Perry,  which  consists  in  replacing  the 
straight  wire  by  a  spiral  made  of  a  flattened  wire,  or  more  simply 
the  use  of  a  straight  wire  flattened  by  a  passage  between  rollers. 

The  first  method  offers  the  greatest  security  from  shocks;  it 
is,  on  the  other  hand,  more  difficultly  realizable;  minute  precau- 
tions should  be  taken  to  prevent  any  rubbing  between  adjoining 


THERMOELECTRIC   PYROMETER  123 

spirals.  The  second  method  allows  more  easily  having  the  large 
angular  displacements  which  are  indispensable  when  it  is  desired 
to  take  readings  upon  a  dial. 

The  most  essential  property  necessary  for  the  wires  is  absence 
of  permanent  torsion  during  the  operations.  These  torsions 
cause  changes  of  zero  which  may  render  worthless  all  the  obser- 
vations if  account  is  not  taken  of  this,  which  complicates  matters 
considerably  if  such  correction  has  to  be  made.  This  result  is 
reached  by  using  wires  as  long  as  possible,  having  not  less  than 
100  mm.  length,  and  by  avoiding  giving  to  them  an  initial  torsion, 
a  precaution  that  should  be  kept  constantly  in  mind,  which  it 
often  is  not.  When  one  wishes  to  adjust  the  coil  to  the  zero  of 
graduation,  one  turns  often  haphazard  either  one  of  the  wires; 
it  may  be  then  that  each  of  the  wires  is  given  an  initial  torsion 
of  considerable  magnitude  and  of  opposite  sign.  If  the  two  wires 
are  not  symmetrical,  as  is  ordinarily  the  case,  the  "permanent 
deformation  resulting  from  this  exaggerated  torsion  will  cause  a 
continual  displacement  of  the  zero  which  may  last  for  weeks  and 
months,  increasing  or  decreasing  during  the  observations  accord- 
ing to  the  direction  of  displacement  of  the  coil.  This  torsion  is 
easy  to  obviate  at  the  time  of  construction,  but  it  is  not  possible 
to  verify  later  its  absence  in  the  case  of  round  wires  or  spirals 
except  by  dismounting  the  apparatus.  On  the  contrary,  by  the 
use  of  stretched  flat  wires  it  is  very  easy  upon  simple  examina- 
tion to  determine  the  existence  or  absence  of  torsion.  This  is 
another  reason  for  employing  them. 

Finally,  use  must  be  made  of  wires  having  a  very  high  elastic 
limit.  For  that  it  is  necessary  that  the  metal  has  been  hardened, 
and  besides  that  the  metal  does  not  undergo  spontaneous  hard- 
ening at  ordinary  temperatures.  Silver,  generally  employed  as 
suspension  wire,  is  worthless.  A  metal,  as  iron,  which  even  after 
annealing  possesses  a  high  elastic  limit,  would  be  perfect  if  it  were 
not  for  its  too  great  alterability.  One  cannot  be  sure  of  having 
uniform  hardening,  because  the  soldering  of  wires,  indispensable 
to  assure  good  contacts,  anneals  them  throughout  a  certain 
length.  German  silver  is  the  metal  the  most  frequently  used 


124  HIGH  TEMPERATURES 

in  galvanometer  suspensions  destined  for  pyrometric  measure- 
ments. The  alloy  of  platinum  with  10  per  cent  of  nickel  seems 
preferable;  after  annealing  it  has  a  high  elastic  limit,  and  possesses 
a  tenacity  much  higher  than  that  of  German  silver.  Its  dis- 
advantage is  to  possess  a  limit  of  elasticity  twice  as  great,  which 
reduces  by  one-half  the  deflections  of  a  given  cross  section  of 
wire.  Phosphor  bronze  also  gives  good  results. 

4.  Installation  of  the  apparatus  for  the  galvanometers,  in  which 
the  coil  is  carried  by  two  opposed  stretched  wires,  necessitates 
special  precautions. 

In  the  first  place,  it  should  be  located  beyond  the  influence  of 
jarrings  of  the  ground,  which  render  reading  impossible;  then  it 
is  necessary  that  its  position  remain  rigorously  fixed.  If,  in  fact, 
the  two  extreme  points  of  suspension  of  the  wires  are  not  exactly 
in  the  same  vertical,  and  if  the  center  of  gravity  of  the  coil  is  not 
exactly  in  the  line  of  the  two  points  of  suspension,  two  conditions 
which  can  be  never  rigorously  realized,  the  apparatus  constitutes 
a  bifilar  pendulum  of  great  sensibility.  .  The  slightest  jarring 
suffices  to  provoke  very  considerable  angular  displacements  of 
the  coil.  To  avoid  them,  the  apparatus  should  rest  upon  a 
metallic  support  attached  to  a  wall  of  masonry.  When  the 
apparatus  is  placed,  as  is  often  the  case,  upon  a  wooden  table 
resting  upon  an  ordinary  wooden  floor,  in  order  to  obtain  a 
deflection  of  the  coil,  and  in  consequence  a  displacement  of  the 
zero,  it  suffices  to  walk  around  the  table,  which  causes  the  floor 
to  bend  slightly,  or  to  provoke  a  current  of  air,  which,  in  chang- 
ing the  hygrometric  state  of  the  legs  of  the  table,  causes  it  to 
tip  somewhat. 

Coils  freely  suspended  from  above  have  not  these  disadvan- 
tages. 

Types  of  Suspended-coil  Galvanometer.  —  A  series  of  galvanom- 
eters have  been  studied  especially  in  view  of  pyrometric  meas- 
urements; we  shall  pass  them  rapidly  in  review. 

For  laboratory  researches  the  usual  swinging-coil  galvanom- 
eter as  made  by  Carpentier  is  often  used  in  France.  One  must 
make  sure  that  these  instruments  satisfy  well  the  indispensable 


THERMOELECTRIC  PYROMETER 


125 


conditions  which  we  have  mentioned,  which  is  not  always  the 
case  when  these  instruments  have  been  constructed  with  refer- 
ence to  the  ordinary  experiments  of  physics. 

This  laboratory  apparatus,  the  only  one  which  existed  at  the 
time  of  the  first  investigations  of  Le  Chatelier,  was  not  trans- 
portable, and  could  not  be  arranged  for  experiments  in  industrial 
works.  It  was  then  necessary  to  devise  a  special  model  of  gal- 
vanometer easy  to  carry  about  and  to  put  in  place.  The  appara- 
tus (Fig.  26)  is  composed  of  two  parts,  the  galvanometer  and  the 
transparent  scale  with  its  light.  The  two  parts  are  symmetrical,. 


Fig.  26.    Le  Chatelier's  Thermoelectric  Galvanometer. 

and,  for  transportation,  may  be  fixed  back  to  back  on  the  same 
plank  carrying  a  handle.  For  observations  they  are  fastened  to  a 
wall  by  means  of  two  nails  driven  in  at  a  suitable  distance  apart. 
The  suspension  wires,  in  case  of  breakage,  may  be  immediately 
replaced.  They  carry,  soldered  to  their  two  ends,  small  nickel 
spheres,  which  one  has  only  to  slip  on  to  forked  pieces  attached 
to  the  top  and  bottom  of  the  coil,  and  to  the  supports  of  the 
apparatus,  respectively.  The  mirror  consists  of  a  plano-convex 
lens,  silvered  on  the  plane  face,  which  gives  much  sharper  and 
brighter  images  than  the  ordinary  small  mirrors  with  parallel 
faces. 
Carpentier  has  also  made  for  the  same  purpose  a  galvanometer 


126  HIGH  TEMPERATURES 

in  which  the  readings  are  taken  by  means  of  a  microscope.  It 
is  an  easily  transportable  apparatus  and  very  convenient.  It 
has  the  fault  to  be  subject  to  a  displacement  of  the  zero  resulting 
from  the  unsymmetrical  heating  of  the  body  of  the  microscope 
by  the  small  lamp  which  lights  the  reticule.  The  stretched  wires 
are  replaced  by  large  spirals  which  offer  an  absolute  resistance  to 
rupture  by  shock  during  transportation. 


Fig.  27.     Keiser  and  Schmidt  Outfit. 

The  use  of  this  apparatus  necessitates  an  arrangement  which 
permits,  during  the  observations,  putting  the  galvanometer  on 
open  circuit  so  as  to  verify  the  zero  reading. 

In  the  three  preceding  galvanometers  the  measurement  of  the 
deflection  of  the  coil  is  made  by  optical  means;  in  the  following, 
the  measurement  is  made  by  means  of  a  needle  swinging  over  a 
scale. 

After  a  study  made  by  Holborn  and  Wien  at  the  Physikalische 


THERMOELECTRIC  PYROMETER  127 

Reichsanstalt  in  Berlin  of  the  Le  Chatelier  thermoelectric  pyrom- 
eter, the  firm  of  Keiser  and  Schmidt  devised  a  needle  galvanom- 
eter (Fig.  27)  which  works  fairly  well,  although  the  early  forms 
of  this  instrument  were  of  too  low  resistance  for  many  industrial 
purposes  and  its  temperature  coefficient  is  unduly  high.  It  has 
the  disadvantage  of  being  somewhat  fragile.  The  suspending 
wire  of  the  coil  does  not  seem  to  have  more  than  ^V  mm-  diameter ; 
the  mounting  of  the  apparatus  is  quite  complicated.  Repairs 
cannot  readily  be  made  either  in  the  laboratory  or  works. 


Fig.  28.     Siemens  and  Halske  Pyrometer  Galvanometer. 

The  firm  Siemens  and  Halske  has  also  devised  an  excellent 
model  of  needle  galvanometer  suitable  for  temperature  measure- 
ments (Fig.  28).  Its  resistance  is  340  ohms,  or  400  ohms  in  the 
later  forms;  the  scale  has  180  divisions,  each  corresponding  to 
10  microvolts.  There  is  also  a  second  graduation  which  gives 
the  temperature  directly  with  the  couple  sold  with  the  apparatus. 
Commutators  allow  of  putting  the  apparatus  successively  in 
communication  with  different  thermoelectric  couples,  if  it  is 
desired  to  take  simultaneously  several  sets  of  observations. 
This  instrument  is  provided  with  a  good  level,  and  has  a  small 
temperature  coefficient.  Hartmann  and  Braun  also  manufac- 


128  HIGH  TEMPERATURES 

ture  excellent  instruments  of  this  type.  Their  wall  pattern  is 
shown  in  Fig.  29. 

Pellin  of  Paris  has  made,  from  designs  of  Le  Chatelier,  a 
needle  galvanometer  of  simple  construction  which  can  be  repaired 
where  it  stands.  The  very  long  suspension  wire  is  of  10  per 
cent  nickel  platinum;  it  has  TV  mm-  diameter  and  is  drawn  out 
flat. 

The  lower  wire  is  made  of  a  spiral  of  the  same  wire  of  ^  mm. 
diameter,  which  is  situated  in  the  interior  of  the  iron  core  so  as  to 


Fig.  29.     Hartmann  and  Braun  Wall  Type. 

insure  uniformity  of  temperature.  When  the  spirals  of  the  sus- 
pension are  unequally  heated  by  radiation  from  the  room  or  for 
other  reason,  there  results  considerable  displacement  of  the  zero. 
A  spirit  level  permits  of  rendering  the  apparatus  vertical,  but  it 
is  prudent,  by  reason  of  the  length  of  the  suspension  wire,  to 
make  sure  directly  of  the  absence  of  rubbing  on  the  coil.  For 
this  a  slight  jar  is  given  to  the  apparatus;  the  point  of  the  needle 
should  take  up  and  keep  for  a  long  time  a  slow  oscillatory  move- 
ment in  the  direction  of  its  length;  the  transverse  oscillations 
ceasing  rapidly  indicate  friction  upon  the  coil. 


THERMOELECTRIC  PYROMETER 


129 


Fig.  30.     Paul's  Unipivot 
Mounting. 


v  Pivot  Galvanometers.  —  The  development  of  satisfactory  piv- 
oted moving-coil  electrical  instruments  with  spring  control, 
whose  indications  as  given  by  a  pointer 
on  a  scale  do  not  change  with  time,  is 
very  largely  due  to  Weston.  It  is  only 
recently,  however,  that  pivoted  milli- 
voltmeters  of  sufficiently  high  resist- 
ance and  range  to  use  with  platinum 
couples  have  been  made.  The  charac- 
teristics of  the  design  of  one  type  of 
such  instruments  are  shown  in  Fig.  30, 
illustrating  Paul's  mono-pivot  construc- 
tion. 

The  indicator  is  a  low-reading  mov- 
ing-coil voltmeter, :  the  circular  coil  of 
which  is  pivoted  at  the  center  of  a 
spherical  iron  core,  and  carefully  balanced  so  that  its  position  is 
unaffected  by  vibration  or  want  of  exact  leveling.  The  pivot 
works  in  a  finely  polished  jewel,  from  which  it  is  completely  lifted 
on  depressing  a  plunger  projecting  through  the  top  of  the  instru- 
ment, thus  rendering  the  apparatus  proof  against  rough  handling 
in  transit.  A  moving  coil  of  low  resistance  is  used  in  conjunc- 
tion with  a  large  resistance  of  negligible  temperature  coefficient 
included  in  the  instrument,  "so  that  any  error  due  to  change  of 
temperature  of  the  indicator  is  thus  reduced  to  a  minimum. 
The  movement  of  the  coil  is  controlled  by  a  spring,  and  the 
index  may  be  set  to  zero,  should  this  be  necessary,  without 
opening  the  instrument,  an  external  adjustment  being  provided 
for  this  purpose.  The  unipivot  principle  entirely  eliminates  the 
delicate  suspensions  previously  used,  which  frequently  caused 
trouble  by  accidental  breakage,  this  necessitating  the  entire  re- 
adjustment of  the  apparatus. 

In  general,  we  may  point  out  that  for  this  type  of  galvanom- 
eter the  case  must  be  dust-free  to  avoid  the  collection  of  particles 
in  the  very  small  clearance  of  the  moving  coil,  mounted  between 
the  pole  pieces  of  a  powerful  permanent  magnet.  When  well 


130  HIGH  TEMPERATURES 

designed  the  magnetic  circuit  does  not  change  its  qualities  appre- 
ciably in  years,  and  such  instruments  are  very  little  affected  by 
extraneous  magnetic  fields,  and  are  very  robust,  being  capable 
of  standing  relatively  rough  handling.  They  require  no  leveling 
and  several  types  have  no  adjustment  whatever.  It  is  usually 
well,  however,  to  have  a  set  screw  to  lock  the  pointer  or  coil 
system,  unless,  as  is  sometimes  the  case,  lifting  the  instrument 
clamps  the  pointer.  It  is  also  convenient  to  be  able  to  adjust 
readily  the  zero  position  of  the  galvanometer,  and  also  to  be  able 
to  eliminate  mechanically  the  effects  of  temperature  change  on 
the  readings  of  such  an  instrument.  There  are  many  milli- 
voltmeters  on  the  market  of  sufficient  range  and  sensibility  for 
the  thermoelectric  measurement  of  temperature,  but  only  a  very 
few  of  them  are  properly  designed  for  such  usage,  and  great  care 
should  be  taken,  in  purchasing  a  pivot  galvanometer,  to  find  out 
if  the  instrument  in  question  is  suited  for  the  work  in  hand.  It 
has  been  the  custom  of  some  dealers  in  pyrometric  apparatus  to 
make  use,  for  example,  of  pivot  milli voltmeters  of  absurdly  low 
resistance  in  connection  with  relatively  high  resistance  thermo- 
couples (see  page  119).  A  milli  voltmeter  may,  therefore,  be 
suitable  for  use  with  one  type  of  thermocouple  and  not  with 
another. 

In  order  to  enjoy  the  practical  conveniences  of  the  pivot  type 
of  galvanometer,  at  least  when  using  platinum  thermocouples, 
some  sacrifice  of  precision,  range  or  sensibility  has  to  be  made. 
It  appears  to  be  as  yet  impracticable,  for  example,  to  make  open- 
scale  instruments  with  a  range  of  18  millivolts  and  increase  the 
resistance  above  170  ohms,  and  the  range  of  the  best  makes  is 
from  90  to  1 60  ohms.  In  this  case,  as  we  have  seen,  the  read- 
ing of  the  galvanometer  will  depend  somewhat  upon  the  length 
of  leads  and  upon  the  depth  of  immersion  of  the  couple  in  the 
heated  space. 

There  are  a  great  many  manufacturers  of  low-resistance  pivot 
millivoltmeters,  some  of  which  are  suitable  for  use  with  base- 
metal  couples  of  sufficiently  low  resistance.  Among  the  manu- 
facturers of  pivot  instruments  suitable  for  use  with  platinum 


THERMOELECTRIC  PYROMETER  131 

couples  are  Paul  of  London,  Siemens  and  Halske,  and  the  Cam- 
bridge Scientific  Instrument  Company.  The  first  makes  a  uni- 
pivot  galvanometer,  and  the  others  double-pivot  instruments  of 
the  West  on  type. 

Temperature  Coefficient  of  Galvanometers.  —  It  is  desirable  that 
the  readings  of  indicating  galvanometers  be  as  little  affected  as 
possible  by  temperature  changes  in  the  instruments  themselves. 
In  the  earlier  pyrometer  galvanometers  this  matter  was  generally 
overlooked,  but  in  many  of  the  newer  instruments  provision  is 
made  for  eliminating  this  effect.  Some  of  the  instruments  most 
commonly  used  in  pyrometric  practice  have  temperature  coeffi- 
cients ranging  from  0.03  per  cent  to  0.25  per  cent  per  degree  C., 
depending  on  the  type  and  maker.  They  all  read  too  low  for 
an  increase  in  temperature.  That  this  is  a  serious  source  of 
error  is  evident  from  an  example.  If  an  instrument  having  a 
temperature  coefficient  of  o.i  per  cent  per  degree  C.  is  calibrated 
at  15°  C.  and  is  used  at  35°  C.,  as  may  readily  happen  in  practice, 
its  readings  will  be  low  by  2  per  cent,  or  20°  C.  at  1000°  C.,  due 
to  this  cause  alone. 

The  simplest  method  in  theory  for  the  elimination  of  this  effect 
is  to  use  wire  having  no  temperature  coefficient,  such  as  man- 
ganin,  for  the  coil  and  auxiliary  resistance  of  pyrometer  galva- 
nometers. Manganin  has  the  further  advantage  that  its  Peltier 
effect  against  copper  is  almost  zero.  It  appears,  however,  to  be 
difficult  to  get  sufficient  sensibility  in  this  way  due  to  the  high 
specific  resistance  of  manganin. 

There  are  various  other  devices  for  cutting  down  or  eliminat- 
ing this  effect,  some  based  on  the  choice  of  materials  for  and 
the  ratio  of  the  coil  and  balance  resistances,  and  others  on  the 
variation  of  the  strength  of  the  magnetic  field  between  the  pole 
pieces,  effected  either  by  hand  or  automatically. 

In  the  single-pivot  indicators  of  100  ohms  total  resistance,  of 
R.  W.  Paul  of  London,  for  example,  the  resistance  of  the  copper 
moving  coil  is  only  10  ohms,  the  balance  being  of  manganin, 
reducing  the  temperature  variation  in  the  resistance  of  this  gal- 
vanometer to  the  order  of  0.047  Per  degree  C. 


132  HIGH  TEMPERATURES 

The  use  of  an  adjustable  magnetic  shunt  for  the  elimination 
of  this  temperature  correction  may  be  illustrated  as  follows: 
The  deflection  D  of  the  galvanometer  may  be  considered  pro- 
portional to  the  product  of  the  flux  F  of  the  magnet  by  that,  /, 
of  the  moving  coil,  or  D  =  kFf.  But  /  is  directly  proportional 
to  the  electromotive  force  e  to  be  measured  and  inversely  to  the 
resistance  of  the  circuit,  whence 


fu  [!+«(<  -IS)]' 

where  ^15  is  the  resistance  of  the  circuit  at  15°  C.,  a  its  tempera- 
ture coefficient,  and  t  its  temperature.     We  have,  therefore, 


D  =  kk'F 


[i+a(/-  15)] 

Since  F  remains  sensibly  constant  with  temperature,  it  follows 
that  in  order  to  have  the  same  deflection  for  a  given  value  of  ey 
it  is  sufficient  to  cause  F  to  change  proportionally  with  the  re- 
sistance of  the  circuit. 

This  is  realized  in  practice,  as  in  the  instruments  of  Chauvin 
and  Arnoux,  by  the  use  of  a  small  bar  of  soft  iron  which  may  be 
brought  nearer  to  or  farther  from  the  poles  of  the  magnet,  which 
operation  produces  a  change  in  the  magnetic  flux  through  the 
movable  coil.  The  motion  of  the  iron  bar  is  controlled  by  a 
screw  whose  head  is  graduated  in  degrees  of  temperature.  The 
temperature  of  the  auxiliary  thermometer  embedded  in  the  gal- 
vanometer case  is  read  and  the  magnet-control  screw  set  to  the 
indicated  temperature,  when  the  galvanometer  readings  are  then 
corrected  for  temperature  coefficient. 

An  automatic  magnetic  balancing  of  the  increase  in  resistance 
of  the  galvanometer  coil  with  temperature  has  been  introduced 
into  the  Thwing  galvanometers,  as  shown  in  Fig.  31.  The  coil 
rotates  about  one  of  its  ends  in  a  uniform  field  between  two  plane 
pole  pieces.  The  two  magnets  that  are  connected  in  parallel  by 
these  pole  pieces  differ  from  those  ordinarily  used  in  being  thin 
and  therefore  flexible.  These  magnets  are  pressed  together 
somewhat  by  the  long  arm  of  a  strong  lever,  the  short  arm  of 


THERMOELECTRIC  PYROMETER 


133 


which  rests  upon  a  post  which  is  part  of  the  aluminium  case. 
The  fulcrum  is  a  bar  of  invar.  Changes  in  temperature  expand 
or  contract  the  aluminium  part,  closing  or  opening  somewhat 
the  gap  between  the  poles  of  the  magnet,  and  the  whole  may  be 
adjusted  so  that  the  change  in  flux  through  the  coil  may  balance 
its  change  in  resistance. 


Dial        Needier, 


y  Lever 


Fig.  31.    Thwing's  Compensating  Device. 

The  Siemens  and  Halske  method  of  temperature  compensation 
is  by  a  suitable  combination  of  series  and  shunted  resistances  of 
copper  and  manganin  in  the  swamping  resistance  of  the  instru- 
ment. 

It  should  perhaps  be  emphasized  at  this  point  that  the  elimi- 
nation of  the  temperature  coefficient  of  the  indicating  galva- 
nometer does  not  do  away  with  making  proper  corrections  for 
changes  in  temperature  of  the  cold  junctions  of  the  thermo- 
couple (see  page  155). 

Galvanometer  Requirements  for  Industrial  Practice.  —  In  many 
industrial  operations  it  is  desirable  to  be  sure  of  temperature 
measurements  to  within  10°  C.,  often  over  a  very  considerable 
temperature  range.  This  accuracy  can  be  obtained  with  certain 
forms  of  the  pyrometer  galvanometer  both  with  platinum  couples 
and  with  some  of  the  base-metal  couples,  but  only  when  certain 
conditions  are  fulfilled  by  the  maker  and  the  user  of  the  instru- 
ment. We  may  emphasize  some  of  the  desirable  and  necessary 
features  of  the  galvanometer,  as  follows: 

The  instrument,  if  of  the  moving-coil  pointer  type,  should  be 
dust-free,  of  sufficient  sensibility  and  range,  and  at  the  same  time 
it  should  have  an  open,  nearly  equidistant  scale  which  is  well 
marked  and  easily  read,  without  parallax,  for  example,  by  means 


134  HIGH  TEMPERATURES 

of  reflection  of  the  pointer  in  a  mirror  alongside  the  scale.  The 
deflection  should  be  aperiodic  or  deadbeat,  and  the  open-circuit 
reading  should  remain  constant  even  after  large  deflections  long 
maintained.  There  should  be,  in  the  case  of  suspended-coil 
instruments  and  in  some  pivot  types,  a  suitable  leveling  device 
which  has  been  accurately  adjusted,  and  in  these  instruments 
particularly  the  case  and  other  supporting  parts  should  be  free 
from  warping.  The  zero  position  of  the  pointer  should  be 
readily  adjustable.  The  instrument  should  either  be  free  from 
errors  due  to  changes  in  its  temperature  or  else  some  form  of 
compensation  provided,  and  there  should  be  no  possibility  of 
thermoelectric  effects  in  the  wiring  within  the  instrument.  The 
effects  of  jarring  due  even  to  shocks  of  considerable  intensity 
and  changes  in  the  surrounding  magnetic  field  should  be  with- 
out material  influence  on  the  readings.  For  pivot  instruments 
particularly,  it  should  be  noted  that  the  same  E.M.F.  always 
gives  the  same  deflections.  Finally,  as  we  have  before  stated,  the 
resistance  of  the  galvanometer  must  be  sufficiently  high  for  the 
type  of  couple  with  which  it  is  to  be  used.  The  effect  of  varia- 
tions in  the  temperature  of  the  cold  ends  of  the  thermocouple 
will  be  treated  later.  When  a  new  couple  is  substituted,  it 
should  be  noted  that  the  E.M.F.  scale  of  the  galvanometer  will 
still  be  correct,  barring  the  effect  of  change  in  resistance  of  the 
circuit,  but  unless  the  new  couple  is  identical  in  its  electrical 
properties  with  the  old,  the  temperature  scale  of  the  instrument 
will  no  longer  hold. 

One  instrument  may  often  serve  for  use  with  several  couples 
of  the  same  or  of  different  types.  It  is  then  very  important 
to  avoid  bad  contacts  in  switches,  and  with  very  low-resistance 
outfits  considerable  errors  that  are  not  readily  detectable  may 
creep  into  the  measurements.  A  galvanometer  suitable  for  use 
with  a  Pt-Rh  couple  may  very  properly  be  used  with  a  low- 
resistance  base-metal  couple  of  higher  E.M.F.  by  putting  addi- 
tional resistance  into  the  circuit  if  necessary,  but  a  galvanometer 
suitable  for  use  with  the  base-metal  couple  may  be  totally  unfit 
for  use  with  one  of  Pt-Rh. 


THERMOELECTRIC  PYROMETER  135 

The  Galvanometer  Method  in  the  Laboratory.  —  On  account 
apparently  of  its  relatively  low  cost,  and  also  because  of  its 
speed  of  operation,  the  galvanometer  method  of  measuring 
temperatures  with  the  thermocouple  has  been  used  frequently 
in  scientific  investigations  of  considerable  delicacy.  It  should 
be  borne  in  mind,  however,  that,  even  with  the  best  pointer 
instruments  carefully  calibrated,  which  are  much  used  in  metal- 
lurgical and  physiochemical  researches,  an  accuracy  of  5°  is 
barely  attainable  with  Pt-Rh  couples,  and  this  only  by  paying 
attention  to  the  numerous  sources  of  error  we  have  emphasized 
above. 

A  sensitive  d'Arsonval  galvanometer  read  by  reflection  upon 
a  straight  graduated  scale,  or  by  means  of  a  telescope  and  scale, 
has  also  been  a  favorite  method  of  working.  In  this  way  the 
sensitiveness  over  the  pointer  method  may  be  increased  greatly, 
but  in  general  the  accuracy  will  not  be  very  materially  improved, 
as  practically  all  of  the  troubles  inherent  to  the  galvanometer 
method  are  usually  still  present,  whatever  the  method  adopted 
for  reading  the  deflection  of  the  galvanometer  coil.  By  slight 
modifications,  the  exactness  of  the  galvanometer  method  may 
be  increased,  as  for  instance  keeping  the  cold  junctions  at  a 
definite  and  known  high  temperature  and  depending  on  the 
sensitive  galvanometer  for  a  smaller  temperature  interval;  or 
better,  by  opposing  the  greater  part  of  the  E.M.F.  of  the  couple 
with  a  known  E.M.F.  furnished  by  a  standard  cell  and  resistance 
or  volt  box.  This  last,  however.,  is  the  simplest  case  of  the 
potentiometric  methods  which  we  shall  now  study. 

Potentiometric  Methods.  —  The  fundamental  principle  on 
which  the  many  potentiometric  methods  are  based  is  the  adjust- 
ing of  the  electric  circuit  so  that  no  current  flows  through  the 
thermocouple.  This  is  accomplished  by  balancing  the  E.M.F. 
generated  in  the  thermocouple  by  an  E.M.F.  whose  numerical 
value  may  be  varied  at  will  and  measured.  Since  the  two 
E.M.F.  Js  are  in  opposition,  the  measurements  may  be  made  to 
have  all  the  advantages  of  a  null  or  zero  method,  which  is  usually 
desirable  in  precision  work. 


136  HIGH  TEMPERATURES 

Apparatus  Required.  —  A  complete  installation  for  work  to 
i°  C.  consists  of: 

i.  A  standard  cell,  which  should  not  have  any  current  pass 
through  it,  and  serves  to  determine,  as  term  of  comparison,  a 
difference  of  potential  between  two  points  of  a  circuit  through 
which  there  is  a  current  given  by  an  accumulator.  The  cell  used 
may  be  a  Latimer-Clark,  whose  electromotive  force  for  small 
changes  in  temperature  is 

e  =  1.433  v°lts  —  0.00119  (t°  —  15°). 

This  cell  is  made  up  as  follows :  zinc,  sulphate  of  zinc,  mercurous 
sulphate,  mercury.  The  zinc  sulphate  should  be  perfectly  neu- 
tral; for  that,  the  saturated  solution  of  the  salt  is  heated  to  40° 
or  more  with  an  excess  of  zinc  oxide  to  saturate  the  free  acid,  is 
then  treated  with  mercurous  sulphate  to  remove  the  excess  of 
zinc  oxide  dissolved  in  the  sulphate,  and  finally  crystallization 
is  produced  at  o°;  one  thus  obtains  crystals  of  zinc  sulphate 
which  can  be  immediately  used. 

This  element  is  very  constant.  With  a  surface  of  zinc  electrode 
equal  to  100  sq.  cm.  and  a  resistance  of  1000  ohms,  the  dropping 
off  of  the  electromotive  force  of  the  cell  in  action  does  not  reach 
nnnr»  with  100  ohms  only,  this  would  be  %fa.  Practically  it  is 
possible,  with  a  resistance  of  1000  ohms,  to  limit  the  surface  of 
the  electrodes  to  30  sq.  cm.,  and  to  do  away  with  the  use  of 
accumulators.  But  then  the  theoretical  advantage  of  the  abso- 
lute rigor  of  the  method  employed  is  lost. 

There  are  other  forms  of  standard  cell  which  possess  the  ad- 
vantages of  portability  and  small  temperature  coefficient,  ren- 
dering them  better  adapted  for  ordinary  use  than  the  original 
Clark  form.  The  Carhart-Clark  cell  is  made  with  unsaturated 
mercurous  sulphate  and  has  the  E.M.F. 

e  =  1.439  ~  0.00056  (f  -  15°). 

In  the  Weston  normal  cadmium  cell,  which  has  generally  re- 
placed the  Clark  as  a  standard  of  E.M.F. ,  and  has  been  officially 
recognized  as  the  standard  by  the  London  Electrical  Conference 
of  1908,  cadmium  and  cadmium  sulphate  replace  the  zinc  and 


THERMOELECTRIC  PYROMETER  137 

zinc  sulphate  of  the  Clark  cell;  its  E.M.F.  at  20°  C.  is  1.0183 
and  its  temperature  coefficient  to  two  terms  as  found  by  Wolff  is: 

Et  =  £20  —  0.04406  (/  —  20°)  —  o.o695  (*  ~  2°)2- 

In  the  portable  form  of  the  cell  the  cadmium  sulphate  is  unsatu- 
rated.  This  portable  cell  has  no  appreciable  temperature  coeffi- 
cient, so  that  no  precautions  as  to  temperature  control  have 
to  be  taken.  This  cell  also  recovers  rapidly  after  maltreatment. 
Its  E.M.F.  is  1.0187  volts  at  20°  C.,  although  different  cells  will 
differ  slightly,  i.e.,  by  ±0.0005  volt.  Hulett  has  tried  using  a 
large-area  cadmium  cell  simultaneously  as  a  battery  and  stand- 
ard E.M.F.  with  considerable  success. 

The  values  of  the  E.M.F.'s  given  above  are  in  international 
volts,  which  are  legal  in  the  United  States  and  used  by  the 
National  Bureau  of  Standards,  and  are  the  values  effective  Jan.  i, 
1911,  as  recommended  by  the  International  Committee  on  elec- 
trical standards.  The  values  previously  used  for  the  Clark  were 
1.434  volts  at  15°  C.,  and  for  the  normal  Weston,  1.0189  volts  at 
25°,  in  the  United  States. 

2.  A  resistance  box,  or  one  of  the  forms  of  potentiometer  of 
which  we  shall  treat  immediately.     The  former  includes  a  fixed 
resistance  of  about  1000  ohms  and  a  series  of  resistances  of  o  to 
10  ohms,  permitting  by  their  combinations  to  realize  in  this  in- 
terval resistances  varying  by  tenths  of  an  ohm.     One  may,  for 
greater  simplicity,  but  by  sacrificing  precision,  replace  this  series 
of  small  resistances  by  a  single  Pouillet's  rheostat  having  a  total 
resistance  of  10  ohms.     This  apparatus  consists  of  two  parallel 
wires  of  a  meter  in  length  and  3  mm.  in  diameter,  made  of  an 
alloy  of  platinum  and  3  per  cent  copper. 

3.  A  sensitive  galvanometer  giving  an  appreciable  deflection  for 
10  microvolts.     Since  it  is  placed  in  the  circuit  of  the  couple,  and 
since  this  is  a  case  of  reduction  to  zero,  use  may  be  made  here  of 
a  Deprez-d' Arson val  galvanometer  of  small  resistance. 

Principle  of  the  Method.  —  If  we  have  an  electric  circuit  con- 
sisting of  a  standard  cell,  or  other  source  of  E.M.F.  of  known 
value  £,  and  a  suitable  combination  of  resistances  whose  total 


138 


HIGH  TEMPERATURES 


value  is  R  for  the  whole  circuit;  and  if  the  thermocouple  in  series 
with  a  galvanometer  is  connected  across  a  portion  r  of  R  so  that 
there  is  no  deflection  of  the  galvanometer,  the  E.M.F.  of  the 
couple  is  given  by  the  expression 


A  modification  of  this  method  eliminating  the  standard  cell 
in  actual  work  with  the  couple  has  its  advantages.  A  storage 
cell  at  W  (Fig.  32)  is  in  series  with  a  rheostat  R  and  a  series 
of  coils  or  combinations  of  coils  and  bridge  wire  represented  by 
AB.  The  E.M.F.  of  the  standard  cell  at  E  is  balanced  against 


Fig.  32.     Principle  of  Potentiometer. 

that  of  the  battery  W  by  varying  R,  the  points  of  contact  M 
and  M '  being  at  A  and  B  and  the  balance  indicated  by  no  current 
in  the  galvanometer.  The  standard  cell  is  now  replaced  at  E 
by  the  couple  whose  E.M.F.  is  to  be  measured;  M  and  M'  are 
then  varied  in  position  until  a  balance  is  again  obtained;  then 

MM' 


e  =  E 


AB 


This  is  the  simplest  form  of  potentiometer,  of  which  there  are 
many  convenient  forms  now  available  for  temperature  measure- 
ments. 

Another  modification  of  this  method,  eliminating  the  use  of  a 
potentiometer  or  carefully  calibrated  resistance  box,  but  requir- 
ing a  calibrated  milliammeter  and  one  or  more  well-known  re- 


THERMOELECTRIC  PYROMETER  139 

sistances,  was  first  used  by  Holman  in  thermoelectric  work,  and 
Fig.  33  illustrates  the  principle.  If  is  a  milliammeter  and  r  a 
small  (o.i  co)  known  resistance,  R  a  rheostat  with  fine  adjust- 
ment, G  the  galvanometer,  and  T  the  thermocouple.  The  de- 
flection of  G  is  brought  to  zero  by  varying  R  when  the  product 
of  the  current  given  by  M  and  the  resistance  r  gives  the  desired 
E.M.F.  With  a  series  of  coils  to  substitute  at  r,  the  range  of 
measurable  temperature  may  be  indefinitely  extended.  The  pre- 
cision of  this  method  is  limited  by  that  of  the  milliammeter  M . 
Siemens  and  Halske  sell  a  convenient  form  of  this  apparatus  as 
devised  by  Lindeck  of  the  Reichsanstalt. 


Fig.  33.    Holman's  Method. 

Various  other  special  forms  of  apparatus  for  the  exact  measure- 
ment of  thermocouple  E.M.F.'s  have  been  devised,  but  they  are 
all  modifications,  more  or  less  complicated,  of  the  above.  We 
shall  treat  of  some  of  them  under  potentiometers. 

Potentiometers  for  Use  with  Thermocouples.  —  Although  the 
galvanometric  method  is  suitable  for  many  technical  thermo- 
electric measurements  of  temperature,  it  is  generally  necessary  to 
resort  to  potentiometric  methods  when  an  accuracy  of  10°  C.  or 
better  is  required,  as  is  the  case  in  many  laboratory  operations. 
This  exact  work  is  usually  best  done  with  thermocouples  of  the 


140 


HIGH  TEMPERATURES 


platinum  metals,  so  that  the  problem  of  best  design  of  poten- 
tiometers for  temperature  measurement  is  quite  a  definite  one. 
The  need  of  sufficiently  sensitive  and  accurate  devices  for  the 
measurement  of  small  E.M.F.'s  in  thermoelectric  pyrometry  has 
acted  as  an  incentive  for  the  great  improvement,  in  recent  years, 
of  apparatus  suitable  for  this  purpose,  and  there  are  now  avail- 
able a  considerable  number  of  potentiometers  meeting  the  re- 
quirements for  very  exact  temperature  measurements  by  this 
method,  as  well  as  less  costly  instruments  giving  an  accuracy 
between  that  obtained  with  the  galvanometer  method  and  the 
more  elaborate  potentiometric  installations. 

R  R' 

BI 1  WWWW VWMM/j 


To  Thermocouple 

scfc 

Fig.  34.     Potentiometer  Indicator  Circuits. 

The  potentiometer  indicator  of  Leeds  and  Northrup,  shown  in 
Fig.  34,  illustrates  a  type  of  instrument  of  intermediate  precision, 
but  without  the  disadvantages  of  the  galvanometric  method,  it 
being  possible  to  get  results  to  about  3°  C.  with  this  apparatus, 
using  Pt-Rh  thermocouples. 

This  indicator  consists  of  a  Weston  standard  cell,  a  secondary 
dry  battery,  and  a  galvanometer  connected  up  as  a  potentiometer, 
the  whole  being  mounted  in  a  box  of  convenient  size,  making  a 
portable  testing  outfit  (Fig.  35).  The  dry  cell  is  continuously 
on  the  closed  potentiometer  circuit  ABCF,  which  includes  the 
two  regulating  rheostats  R  and  R'  and  a  fixed  resistance  S.  The 
current  in  the  potentiometer  circuit  is  adjusted  by  changing  R 


THERMOELECTRIC  PYROMETER  141 

and  R'  with  the  key  at  SC  until  the  galvanometer  shows  no  de- 
flection. Pressing  the  key  to  TC,  the  pointer  G  is  set  on  the 
slide  wire  DE,  calibrated  in  millivolts,  until  again  the  galvanom- 
eter remains  undeflected,  indicating  a  balance  in  the  thermo- 
couple circuit. 

Precision  Requirements.  —  Some  of  the  requirements  which 
must  be  met  in  potentiometer  construction  we  may  emphasize. 
For  work  to  0.1°  C.  with  Pt-Rh  couples,  for  example,  we  must 


Fig.  35.     Potentiometer  Indicator. 

have  a  sensibility  of  i  microvolt  (millionth  of  a  volt)  throughout 
the  range  of  the  instrument,  which  may  be  of  20  millivolts,  neces- 
sitating an  accuracy  of  i  in  20,000  in  all  adjustments  affecting 
the  final  value  of  the  E.M.F.  Contact  or  thermal  E.M.F.'s, 
such  as  develop  even  for  slight  temperature  differences  in  the 
various  parts  of  such  an  apparatus,  are  to  be  avoided  in  the 
electric  circuits,  as  far  as  possible,  by  proper  choice  of  materials, 
design,  and  method  of  manipulation;  for  example,  using  thin 
metal  contacts,  putting  sliding  contacts  in  battery  circuit,  and 
working  with  the  galvanometer  circuit  closed.  In  order  to  in- 


142  HIGH  TEMPERATURES 

crease  the  sensibility  and  permit  the  use  of  moving-coil  galva- 
nometers of  reasonably  attainable  behavior,  it  is  necessary  to 
keep  down  the  total  resistance  of  the  potentiometer.  This  causes 
the  contact  resistances  of  the  adjustable  parts,  such  as  the  dials, 
to  become  of  importance,  and  a  very  exact  and  somewhat  com- 
plicated mechanical  construction  is  required  to  eliminate  this 
source  of  error.  It  appears  to  be  practically  necessary,  in  design- 
ing a  potentiometer,  to  choose  between  some  contact  resistance 
or  some  thermal  E.M.F. 

For  rapid  work,  it  is  desirable  that  the  potentiometer  circuit 
be  so  designed  that  the  standard  cell  may  be  checked  up  without 
disturbing  the  potentiometer  circuit,  and  similarly  it  is  advan- 
tageous to  be  able  to  change  the  range  of  the  potentiometer 
without  disturbing  the  regulating  rheostats  or  rechecking  the 
standard  cell.  Sometimes,  also,  the  final  figure  in  E.M.F.  is 
obtained  from  the  galvanometer  deflection,  in  which  case  it  is 
convenient  to  make  provision  for  a  constant  galvanometer  sen- 
sibility for  all  E.M.F.'s,  which  may  be  effected  by  auxiliary 
resistances  in  the  galvanometer  circuit. 

A  very  important  matter  is  that  of  insulation,  or  the  prevention 
of  leaks  from  one  part  of  the  potentiometer  circuit  to  another 
(internal  leakage)  and  from  the  outside  to  or  from  this  circuit 
(external  leakage).  The  former  becomes  less  important  with 
low-resistance  potentiometers.  The  latter  effect  becomes  par- 
ticularly menacing  when  the  thermocouple  is  immersed  in  an 
electrically  heated  furnace.  It  can  be  overcome  by  interposing 
wire-connected  equipotential  shields  made  of  metal  between 
the  measuring  system  and  all  external  sources  of  E.M.F.,  or  by 
reversal  of  the  heating  or  other  suspected  circuit  and  taking  the 
mean  of  the  potentiometer  readings. 

Most  of  the  potentiometers  in  use  are,  in  part  at  least,  slide- 
wire  instruments,  but  for  the  very  highest  accuracy  it  is  ad- 
visable to  use  the  more  costly  dial  construction  throughout.  As 
we  shall  see,  potentiometers  suitable  for  the  thermoelectric  or 
resistance  measurement  of  temperature  may  now  be  obtained, 
provided  with  five  dials  and  reading  accurately  to  o.i  microvolt, 


THERMOELECTRIC  PYROMETER  143 

or  to  considerably  better  than  any  thermocouple  can  be  depended 
upon  at  high  temperatures. 

An  inconstant  battery  is  troublesome,  and  in  exact  work  it  is 
necessary  to  pay  particular  attention  to  this  point  in  addition 
to  frequently  checking  against  the  standard  cell.  Accumulators 
of  considerable  volume,  or  several  so  connected  as  to  give  a 
minimum  change  of  E.M.F.  with  time,  should  be  used;  and  it  is 
well,  since  so  little  current  is  taken  from  the  battery,  to  have  it 
constantly  closed  through  its  potentiometer  circuit.  The  battery 
may  also  be  advantageously  inclosed  and  packed  to  obviate 
temperature  changes  which  may  cause  fluctuations  in  its  E.M.F. 
of  sufficient  magnitude  to  be  troublesome  in  work  of  high  pre- 
cision. 

Some  care  has  to  be  exercised  in  the  choice  or  design  of  the 
'galvanometer  to  be  used  with  precision  potentiometers.  For 
work  to  0.1°  C.  with  platinum  thermocouples,  it  is  necessary  to 
have  an  appreciable  deflection  for  i  microvolt  with  the  galva- 
nometer in  circuit,  and  the  design  should  be  such  that  the  de- 
flection is  aperiodic  when  the  galvanometer  is  used  with  a  given 
potentiometer.  For  rapid  work,  as  in  taking  cooling  curves,  the 
period  of  the  galvanometer  should  be  kept  down;  and  if,  besides, 
the  last  increment  of  E.M.F.  is  to  be  measured  by  the  galva- 
nometer swing,  it  is  desirable  to  have  a  period  of  not  over 
five  seconds.  These  requirements,  combined  with  freedom  from 
thermoelectric  effects,  are  very  severe  for  the  swinging-coil  type 
of  galvanometer  and  can  be  met  only  by  the  more  skillful  con- 
structors of  such  instruments. 

Types  of  Thermocouple  Potentiometer.  —  The  Cambridge  ther- 
mocouple potentiometer,  similar  in  design  to  that  of  Harker,  is 
an  instrument  designed  for  measuring  E.M.F.'s  of  30  millivolts 
or  less.  By  estimation,  microvolts  may  be  read,  corresponding 
to  about  0.1°  at  1000°  C.  with  Pt-Rh  couples.  The  circuits 
of  this  potentiometer  are  shown  diagrammatically  in  Fig.  36. 
The  total  resistance  in  the  circuit  is  arranged  to  give  a  fall  of 
potential  of  about  i  volt  per  50  ohms,  and  the  resistances  B.C. 
(about  42.5  co)  and  s.c.  (about  51  o>)  are  adjusted  to  give  a  fall  of 


144 


HIGH  TEMPERATURES 


potential  from  M  to  N  on  the  slide  wire  ss  equal  to  the  E.M.F, 
of  a  cadmium  cell  C. 

This  potentiometer  is  operated  as  follows:  With  N  set  at  the 
known  value  of  the  standard  cell  C,  and  the  key  k  thrown  to  ccr 
putting  C  in  opposition  with  the  storage  cell  B,  the  resistances 
RI  and  R2  are  adjusted  until  the  galvanometer  G  shows  no  de- 
flection on  tapping  the  key.  The  battery  B  is  then  substituted 
for  the  cell  C  by  throwing  k  to  the  side  xx  for  the  determination 
of  the  unknown  E.M.F.,  X.  The  balancing  of  X  against  B  is 
made  by  setting  the  dial,  or  series  of  millivolt  coils,  MVCr 
and  the  pointer  Q  on  the  slide  wire  V  V,  until  as  before  the  galva- 
nometer shows  no  deflection  on  pressing  the  key.  The  value 


Fig.  36.     Cambridge, SThehflocouple  Potentiometer.       ^\ 

of  X  is  then  given  directly  in  millivolts  by  adding  the  readings 
of  MVC  and  Q.  This  is  effected  by  making  the  dial  MVC  of 
29  coils  each  of  0.05  co,  giving  on  the  basis  of  i  volt  per  50  ohms 
a  pressure  diagram  of  i  millivolt  on  each  section.  Similarly, 
the  resistance  of  the  wire  VV  being  0.06  ohm,  the  fall  of 
potential  along  its  length  is  1.2  millivolts,  or  the  maximum 
E.M.F.  measurable  is  30.2  millivolts.  This  range  will  take  in 
most  base-metal  thermocouples  as  well  as  the  usual  platinum 
couples.  In  order  to  minimize  thermal  E.M.F. 's  and  tempera- 
ture coefficients,  all  coils  are  of  manganin  and  all  connections 
of  copper. 


THERMOELECTRIC  PYROMETER 


14$ 


The  Leeds  and  Northrup  thermocouple  potentiometer  repre- 
sents another,  if  somewhat  similar,  solution  of  this  problem  to 
about  the  same  degree  of  accuracy.  The  arrangement  of  circuits 
is  shown  in  Fig.  37.  By  means  of  the  plug  at  A  the  range  of 
the  instrument  may  be  increased  tenfold.  The  heavy  slide 
wire  possesses  eleven  turns  and  permits  reading  to  better  than  i 
microvolt  with  a  suitable  galvanometer.  The  resistance  of  each, 
of  the  seventeen  millivolt  coils  is  0.5  ohm,  giving  with  the  slide 
wire  a  total  of  about  9  ohms  in  the  main  circuit. 


Ba.— 


I     !>           i 

0        O       S                 C 

yj~j 

fflCr 

U 

QE.M-F.Q 

OcenO 
Fig.  37.     Leeds  and  Northrup  Thermocouple  Potentiometer. 

In  both  of  the  above  instruments,  settings  on  the  standard  cell 
may  be  made  without  disturbing  the  battery  circuit,  and  the 
range  and  sensibility  of  either  may  be  increased  at  will  by  suit- 
able devices  which  may  conveniently  be  built  into  the  instru- 
ments. There  are  also  numerous  other  potentiometers,  such  as 
those  of  Siemens  and  Halske,  Carpentier,  and  Wolff,  based  on 
similar  methods  of  operation.  This  type  of  instrument  is  not 
entirely  free  from  internal  thermoelectric  forces,  but  these  may 
be  practically  eliminated  by  proper  reversals  in  the  circuits. 

The  Diesselhorst  potentiometer,  built  by  O.  Wolff  of  Berlin, 
is  based  on  quite  different  principles  from  the  preceding  and 


146  HIGH  TEMPERATURES 

represents  an  attempt  to  attain  the  highest  accuracy  possible 
in  such  apparatus,  o.i  microvolt  being  measurable  with  exact- 
ness. It  is  a  five-dial  instrument  of  very  low  resistance,  and 
combines  principles  of  construction  suggested  by  several  writers 
including  Hausrath,  White,  and  Diesselhorst.  Thermoelectric 
effects  in  the  main  potentiometer  circuit  are  eliminated  by  the 
design  of  the  instrument,  and  temperature  coefficient  changes 
in  the  coils  may  be  avoided  by  oil  immersion.  The  effects  of 
contact  resistances  are  eliminated  only  by  the  excellence  of 
construction.  As  constructed,  this  potentiometer  possesses  the 
disadvantages  usually  common  to  split-circuit  instruments  in 
which  the  range  is  altered  by  changing  the?  battery  current, 
such  as  requiring  the  adjustment  of  the  rheostat  in  the  battery 
circuit  and  rebalancing  against  the  standard  cell  whenever  the 
range  of  the  instrument  is  changed.  This  is  prohibitive  for  the 
rapid  intercomparison  of  considerably  different  E.M.F.'s  such 
as  is  often  required  in  temperature  measurements,  unless  sensi- 
bility is  sacrificed. 

White  has  developed  a  dial  potentiometer  suitable  for  thermo- 
electric work  of  high  accuracy,  in  which,  however,  the  last  two 
dials  are  replaced  by  the  galvanometer  deflection,  necessitating 
a  construction,  which  has  been  realized,  giving  constant  galva- 
nometer sensibility.  White  has  also  realized  a  double  potenti- 
ometer permitting  alternate  and  independent  measurements  of 
two  rapidly  varying  E.M.F.'s  with  all  the  advantages  of  two 
instruments,  but  with  the  accessories  of  only  one. 

Finally,  Wenner  has  suggested  a  modification  of  the  potenti- 
ometer circuit  suitable  for  the  measurement  of  low  E.M.F.'s, 
consisting  in  shunting  by  a  comparatively  high  resistance  a  part 
of  the  circuit  including  the  potential  point  of  a  dial.  By  means 
of  a  double-dial  switch  both  branch  points  between  the  shunt 
and  the  main  circuit  may  be  shifted  in  steps  of  equal  resistance 
so  as  to  introduce  a  larger  or  smaller  resistance  in  the  dial  while 
keeping  the  resistance  shunted  constant. 

In  Fig.  38  is  shown  a  plan  for  this  potentiometer  for  use  with 
thermocouples. 


THERMOELECTRIC  PYROMETER 


147 


The  dial  contacts  are  all  in  the  battery  circuit,  each  branch  of 
which  is  of  comparatively  high  resistance,  so  that  the  resistance 
of  the  contacts  and  thermoelectromotive  forces  due  to  the  setting 
of  the  dials  have  only  a  very  small  effect.  The  compensation 
circuit,  on  the  other  hand,  is  of  low  and  nearly  constant  resistance, 
which  makes  it  possible  to  use  a  galvanometer  having  a  high 
voltage  sensibility  and  permits  the  reading  of  a  small  unbalanced 
electromotive  force  from  the  deflection  of  the  galvanometer  (G). 


Fig.  38.     Wenner's  Design. 

The  effect  of  thermoelectromotive  forces  in  the  galvanometer 
is  much  reduced  by  keeping  the  circuit  closed  and  the  resistance 
approximately  independent  of  the  position  of  the  galvanometer 
key.  Under  these  conditions  a  change  in  the  deflection  of  the 
galvanometer  following  a  change  in  the  position  of  the  key  (K) 
signifies  an  uncompensated  electromotive  force  independent  of 
any  fairly  constant  electromotive  force  in  the  galvanometer. 

The  question  of  best  design  of  precision  potentiometers  for 
use  with  thermocouples  may  be  said  to  be  in  a  state  of  flux,  and 
no  single  best  instrument  meeting  satisfactorily  all  the  conditions 
imposed  above  has  yet  appeared  in  practical  form. 

The  Thermocouple  Circuit.  —  For  good  working  of  the  plati- 
num thermocouple  there  are  certain  practical  precautions  to  be 
taken,  which  we  shall  consider.  Most  of  these  remarks  apply 
even  with  greater  force  to  the  base-metal  couples. 


148  HIGH  TEMPERATURES 

Junction  of  the  Wires.  —  The  contacts  of  the  different  parts 
of  the  circuit  should  be  assured  in  a  positive  manner;  the  best 
way  is  to  solder  them.  Binding  screws  often  work  loose  in  time, 
or  the  metallic  surfaces  in  contact  become  oxidized.  The  im- 
portance of  this  precaution  varies  with  the  conditions  of  the 
experiments;  one  can  dispense  with  it  for  experiments  that  last 
only  a  few  hours,  because  there  is  little  chance  that  the  con- 
tacts will  become  modified  in  so  short  a  time;  soldering  is,  on 
the  contrary,  indispensable  in  an  industrial  installation  which 
may  be  used  for  months  without  being  tested  anew. 

But  in  any  case,  the  soldering  together  of  the  two  leads  of 
the  couple  is  absolutely  indispensable.  It  is  quite  true  that  the 
electromotive  force  is  independent  of  the  manner  of  making 
contact.  The  two  wires  twisted  together  or  soldered  will  give 
at  the  same  temperature  the  same  electromotive  force.  But 
under  the  action  of  heat  the  twisted  parts  are  soon  loosened, 
and  there  result  bad  contacts  which  increase  the  resistance  of 
the  whole  circuit.  In  general,  this  accident  is  not  noticed  until 
the  untying  is  almost  complete,  so  that  one  may  make  before 
this  a  whole  series  of  false  measurements  without  being  warned. 

The  best  method  of  soldering  is  the  autogene  junction  by 
direct  fusion  of  the  wires  of  the  couple;  it  is  necessary,  in  order 
to  effect  this,  to  have  oxygen  at  hand.  One  commences  by 
twisting  the  two  leads  together  for  a  length  of  about  5  mm.,  and 
they  are  then  clamped  above  an  oxy hydrogen  blast  lamp.  Oxy- 
gen is  admitted  through  the  central  tube,  and  gas  through  the 
annular  space ;  the  oxygen  is  allowed  to  flow  in  normal  quantity, 
and  the  gas  in  feeble  quantity,  then  one  opens  progressively  the 
gas  cock.  At  a  certain  instant  one  sees  the  extremities  of  the 
wires  melt,  giving  off  sparks;  the  gas  is  then  shut  off.  If  one 
waits  too  long,  the  junction  will  melt  completely  and  the  two 
wires  separate.  With  a  little  practice  a  good  junction  can  be 
made  by  touching  together,  in  the  oxyhydrogen  blast,  the  two 
untwisted  wires  held  in  the  hand. 

In  default  of  oxygen,  the  wires  may  be  soldered  with  palladium, 
which  can  be  melted  by  means  of  a  blast  lamp  furnished  with  air, 


THERMOELECTRIC  PYROMETER  149 

taking  care  to  reduce  the  action  of  radiation.  A  hole  is  cut  in  a 
piece  of  charcoal  in  which  is  placed  the  junction  of  the  two  wires 
twisted  together  after  having  wound  about  it  a  wire  or  a  small 
strip  of  palladium,  and  the  flame  of  the  lamp  is  then  directed 
upon  the  junction. 

In  the  cases  in  which  the  couple  is  not  to  be  used  above  1000°, 
and  only  in  these  cases,  the  soldering  may  be  done  still  more 
simply  by  the  use  of  gold ;  the  ordinary  Bunsen  flame  is  sufficient 
to  make  this  junction. 

Annealing.  —  Before  use,  even  with  new  couples  which  are 
usually  hard-drawn,  the  wires  of  the  couple  should  be  rendered 
as  homogeneous  as  possible  by  annealing  them  electrically.  For 
the  platinum  couples  of  0.6  mm.  diameter,  which  are  in  com- 
mon use,  a  current  of  14  amperes  usually  suffices.  The  cur- 
rent is  kept  on  until  the  wires  glow  uniformly.  In  the  case  of 
couples  that  have  been  used,  bad  spots  are  easily  detected  in 
this  way,  and  should  be  cut  out  if  the  glowing  does  not  remove 
them. 

Insulation  and  Protection.  —  The  two  leads  should  be  insulated 
from  one  another  throughout  their  length.  For  this,  use  may  be 
made  in  the  laboratory  of  glass  tubes  or  pipestems,  or  of  thread 
of  pure  asbestos  wound  about  the  two  wires,  by  crossing  it  each 
time  between  the  two  (Fig.  60)  so  as  to  make  a  double  knot  in 
the  form  of  an  eight,  each  of  the  wires  passing  through  one  of  the 
loops  of  the  eight.  This  is  a  convenient  method  of  insulation  for 
laboratory  use,  although  ordinary  asbestos  is  likely  to  contain 
impurities  which  will  damage  the  couple.  The  two  wires  with 
their  envelope  form  a  small  rod  of  considerable  rigidity  which  is 
easily  slipped  into  apparatus.  With  this  arrangement  it  is  im- 
possible to  go  above  1200°  or  1300°,  at  which  temperature  asbes- 
tos melts.  The  most  satisfactory  insulation,  however,  is  had 
by  means  of  thin  tubes  of  hard  porcelain  standing  1500°  C.  and 
of  Marquardt  mixture,  1600°,  obtained  from  the  Royal  Berlin 
Porcelain  Works. 

For  industrial  installations,  use  may  be  made  of  small  fire-clay 
cylinders  of  100  mm.  in  length  and  10  mm.  in  diameter,  pierced 


HIGH  TEMPERATURES 


Fig.  39.     Parvillee's 
Mounting. 


in  the  direction  of  the  axis  by  two  holes  of  i  mm.  diameter,  through 
which  pass  the  wires,  or  hard  porcelain  tubes  may  be  used. 
One  or  another  of  the  other  forms  of  insula- 
tor is  added  in  sufficient  numbers.  They  are 
placed,  according  to  the  case,  in  an  iron  tube 
or  in  a  porcelain  tube.  The  porcelain  tube 
should  be  employed  in  fixed  installations  in 
which  the  temperatures  may  exceed  800°. 
One  may,  as  does  Parvillee  in  his  porcelain 
furnaces  (Fig.  39),  place  the  porcelain  tube 
in  the  lining  of  the  furnace  in  such  a  way  that 
its  end  is  flush  with  the  inner  surface  of  the 
lining.  An  open  space  of  a  decimeter  cube  is 
cut  in  the  lining  about  this  extremity  of  the 
tube.  This  method  makes  easier  the 
establishment  of  temperature  equi- 
librium without  subjecting  the  tube  to  too  great 
chances  of  breaking  by  accidental  blows. 

The  iron  tube  is  used  for  temperatures  not  exceed- 
ing 800°,  in  the  lead  baths  serving  to  harden  steel  for 
example,  and  for  movable  couples  which  are  exposed 
to  heat  only  during  the  time  necessary  to  take  the 
observations.  In  this  case  the  junction  is  placed 
some  5  cm.  beyond  the  insulators  and  the  iron  jacket. 
The  wires  take  up  the  temperature  within  5  seconds, 
and  the  observation  can  be  taken  before  the  tube 
becomes  hot  enough  to  be  burned,  even  in  furnaces 
for  steel  whose  temperatures  exceed  1600°,  and  before 
the  wires  have  had  time  to  be  altered  even  in  strongly 
reducing  flames.  The  other  extremity  of  the  iron 
tube  carries  a  wooden  handle  (Fig.  40)  where  are 
located,  outside,  the  binding  posts  for  the  galva- 
nometer leads,  and  inside  an  extra  length  of  wire  for 
the  couple  to  replace  portions  burned  or  broken  off. 
The  figure  shows  one  arrangement  of  this  handle. 

In  all  cases  in  which  the  furnace  whose  temperature  it  is  de- 


Fig.  40. 

Opened 
Wooden 
Handle. 


THERMOELECTRIC   PYROMETER 


sired  to  measure  is  under  a  reduced  pressure,  suitable  precautions 
must  be  taken  to  prevent  any  permanent  entrance  of  cold  air  by 
the  orifice  necessary  for  the  intro- 
duction of  the  tube,  as  well  before 
as  during  an  observation;  otherwise 
one  runs  the  chance  of  having  in- 
exact results. 

In  the  case  of  prolonged  observa- 
tions in  a  reducing  atmosphere  or  in 
contact  with  melted  bodies,  as  the 
metals  capable  of  altering  the  plat- 
inum, the  couple  should  be  protected 
by  inclosing  it  in  a  covering  imper- 
meable to  the  melted  metals  and  to 
vapors.  For  fixed  installations  in 
industrial  works,  use  should  be  made 
of  a  porcelain  tube,  or  one  of  iron, 
closed  at  the  extremity  where  the 
junction  is  located;  in  this  case  the 
dimensions  of  the  tube  are  unim- 
portant. Quartz  or  porcelain  tubes 
with  an  iron  tube  furnish  oftentimes 
a  very  permanent  and  satisfactory 
sheathing.  Fig.  41  shows  one  form 
of  mounting  for  a  protected  couple 
attached  to  its  galvanometer.  For 
laboratory  investigations  it  is  often 
indispensable,  on  the  contrary,  to 
have  around  the  wires  a  covering 
of  as  small  diameter  as  possible  If 
it  is  simply  a  question  of  protect- 
ing the  couple  against  the  action 
of  non-volatile  metals,  the  simplest 
way  is  to  use,  as  did  Roberts- 
Austen,  a  paste  sold  in  England  under  the  name  of  Purimachos, 
which  serves  to  repair  the  cazettes  employed  in  molding.  We 


152  HIGH  TEMPERATURES 

have  made  an  analysis  of  this  which  gave  the  following  compo- 
sition after  desiccation  at  200°: 

Alumina  and  iron 14 

Soda 3.2 

Water 2.6 

Silica  (by  difference) 80 . 2 

It  is  a  very  finely  powdered  quartz  to  which  is  added  10  per 
cent  of  clay,  and  diluted  with  a  solution  of  silicate  of  sodium. 
To  use  it,  the  matter  is  diluted  so  as  to  form  a  thick  paste, 
and  the  couple  is  dipped  in  it  the  required  length,  arranging 
the  wires  parallel  to  each  other  at  a  distance  apart  of  about 
i  mm. 

The  whole  may  then  be  dried  and  calcined  very  rapidly,  with- 
out fear  of  snapping  the  covering,  as  would  happen  with  clay 
alone;  but  this  covering  is  not  sufficiently  impermeable  to  protect 
the  couple  against  the  very  volatile  metals,  as  zinc.  It  is  better, 
in  this  case,  to  use  small  porcelain  tubes  of  5  mm.  inside  diameter, 
i  mm.  thickness  of  wall,  and  100  mm.  long,  straight  or  curved, 
according  to  the  usage  to  which  they  are  to  be  put. 

The  couple  insulated  by  asbestos  thread,  or  by  a  small  inner 
porcelain  tube  of  i  mm.  inside  diameter,  as  has  been  said  pre- 
viously, is  pushed  down  to  the  bottom  of  the  tube.  If  one  has 
not  at  hand  such  tubes  of  porcelain,  and  it  is  required  to  make  a 
single  observation  at  a  temperature  not  exceeding  1000°,  as,  for 
instance,  a  standardization  in  boiling  zinc,  one  may  use  a  glass 
tube.  It  melts  and  sticks  to  the  asbestos,  which  holds  a  thick 
enough  layer  to  itself  to  protect  the  platinum.  But,  on  cooling, 
the  tube  breaks,  and  it  is  necessary  to  make  a  new  set-up  for  each 
operation.  This  is  not  practicable  for  continuous  observations. 

Fused  quartz  is  now  obtainable  for  insulating  thermocouples 
and  for  containing  sheaths.  This  material  gradually  crystallizes 
and  crumbles  above  1200°,  and  in  the  presence  of  a  volatile  reduc- 
ing agent,  as  graphite  or  hydrogen,  volatile  silicides  are  formed 
above  1200°  C.,  which  will  destroy  platinum.  Some  types  of 
industrial  mountings  used  by  Heraeus  for  platinum  thermo- 
couples are  shown  in  Fig.  42. 


THERMOELECTRIC  PYROMETER 


153 


orcelain 
Tube 


Metal  Porcelain  Quautz  Glass        Double  Steel  Tube     Graphite 

Fig.  42.    Herseus'  Thermocouple  Mountings. 


154  HIGH  TEMPERATURES 

Cold  Junction.  —  In  general,  in  a  thermoelectric  element,  one 
distinguishes  the  hot  junction  and  the  cold  junction.  The  latter 
is  supposed  kept  at  a  constant  temperature.  In  order  to  realize 
rigorously  this  arrangement,  three  wires  are  necessary,  two  of 
platinum  and  one  of  an  alloy  connecting  two  junctions.  This 
theoretical  arrangement  is  practically  without  interest,  and  the 
second  junction  is  always  dispensed  with.  If,  in  fact,  the  tem- 
perature of  the  whole  circuit  exclusive  of  the  hot  junction  is 
uniform,  the  presence  or  the  absence  of  the  cold  junction 
does  not  affect  the  electromotive  force;  if  this  temperature  is 
not  uniform,  the  second  junction  is  not  advantageous,  for 
there  is  then  in  the  circuit  an  infinity  of  other  junctions  just 
as  important  to  consider:  the  junctions  of  the  copper  leads 
with  the  platinum  wires,  those  of  the  galvanometer  leads 
and  of  the  different  parts  of  the  galvanometer  among  them- 
selves. 

One  must  satisfy  himself  as  well  as  may  be  as  to  the  uniformity 
of  temperature. in  the  cold  circuit,  and  rigorously  of  the  equality 
of  temperature  between  corresponding  junctions,  particularly 
those  of  the  two  platinum  wires  with  the  copper  leads.  These 
uncertainties  in  the  temperature  of  the  cold  junctions  are  an 
important  source  of  error  in  the  measurement  of  temperatures 
by  thermoelectric  couples,  but  for  ordinary  practice  they  are 
easily  eliminated.  In  order  to  realize  exact  measurements,  pre- 
cise to  i°,  for  instance  by  the  galvanometer  method,  it  will 
be  necessary  to  have  completely  homogeneous  circuits,  includ- 
ing the  galvanometer,  with  the  single  exception  of  the  junc- 
tions of  the  platinum  wires  with  the  conducting  leads;  these 
should  be  immersed  in  the  same  bath  at  constant  tempera- 
ture. It  would  be  necessary  for  this  that  the  constructors 
of  galvanometers  limit  themselves  to  the  use  of  the  same 
kind  of  wire  for  all  parts  of  the  apparatus,  wires  of  the  coil, 
suspending  wires,  leads,  and  parts  of  the  coil.  That  is  difficult 
to  obtain. 

In  the  standardization  of  thermocouples  for  exact  work,  it 
is  customary  to  immerse  the  cold  junctions,  i.e.,  the  points 


THERMOELECTRIC  PYROMETER  155 

of  contact  of  the  copper  leads  and  platinum-metal  wires, 
in  an  oil  bath  in  ice.  With  the  potentiometer,  irregularities 
due  to  other  sources  of  E.M.F.  in  the  circuit  are  eliminated 
by  reversing  simultaneously  the  battery  current  and  the  couple 
circuit. 

The  Cold-junction  Correction.  —  In  work  of  high  accuracy 
with  platinum  couples  and  when  the  potentiometric  method  of 
measurement  is  used,  the  cold-junction  correction  should  be 
experimentally  eliminated  by  keeping  the  cold  junctions  at  a 
constant  temperature,  most  conveniently  at  o°  C. 

When  the  galvanometer  method  is  used,  it  is  often  not 
convenient  to  keep  the  junctions  of  the  couple  to  the  lead 
wires  of  the  galvanometer  at  a  definite  temperature,  although 
the  galvanometer  itself  may  be  so  removed  from  the  fur- 
nace that  its  temperature  changes  are  slight.  Except  in  the 
roughest  kind  of  work,  allowance  has  to  be  made  for  the  cold- 
junction  temperatures,  which  may  be  measured  by  an  auxiliary 
thermometer. 

Calling  /0  the  cold-junction  temperature  for  which  the  instru- 
ment reads  correctly,  /  the  observed  temperature  of  the  cold 
junction,  the  correction  to  apply  to  the  observed  temperature 
readings  of  the  galvanometer,  otherwise  supposed  to  read  cor- 
rectly for  a  given  thermocouple,  usually  lies  between  f  (/  —  /0) 
and  (/  —  /o),  depending  on  the  type  of  couple  and  the  tempera- 
tures of  both  hot  and  cold  junctions.  This  question  has  been 
treated  in  detail  for  several  types  of  couple  by  C.  Otterhaus  and 
E.  H.  Fischer. 

That  this  correction  depends  in  general  upon  both  hot  and 
cold  junctions  is  due  to  the  fact  that  the  E.M.F.-temperature 
curve  is  not  a  straight  line  (see  Fig.  24).  The  correction  factor 
by  which  to  multiply  (t  —  to)  is  numerically  equal  to  the  ratio 
of  the  tangents  of  this  curve  for  the  hot-  and  cold-junction 
temperatures. 

As  an  example,  we  may  compute  the  corrections  to  apply  for 
a  Pt,  90  Pt-io  Rh  Heraeus  thermocouple  using  the  E.M.F.  data 
of  Day  and  Sosman  (page  114). 


156  HIGH  TEMPERATURES 

CORRECTIONS  FOR  COLD  JUNCTION  (Pt,  90  Pt-io  Rh). 


Temperature 
of  hot 

Correction  factor  for  cold  junctions  near: 

junction. 

0° 

20° 

40° 

100°  C. 

0.76 

0.81 

0.86 

200 

-65 

.68 

•73 

300 

.60 

•63 

.68 

400 

•57 

.61 

•65 

500 

•55 

•59 

•63 

600 

•54 

•57 

.61 

700 

•53 

•55 

•59 

800 

•54 

•  57 

900 

•49 

•  52 

•55 

IOOO 

•  48 

•50 

•  53 

I2OO 

•  46 

•49 

•  Si 

I4OO 

•45 

-48 

•50 

1600 

•45 

.48 

•50 

It  is  to  be  kept  in  mind  that  the  E.M.F.  indicated  by  a  direct- 
reading  galvanometer  is  a  measure  of  the  difference  in  tempera- 
ture between  the  hot  and  cold  junctions.  If  the  galvanometer 
needle  be  set  at  zero,  which  is  a  convenient  way  of  working,  this 
zero  reading  will  correspond  to  the  temperature  of  the  cold 
junction  at  the  start;  therefore  the  true  temperature  is  obtained 
by  adding  to  the  observed  temperature  reading  a  quantity  corre- 
sponding in  millivolts  to  the  cold-junction  temperature,  obtained 
as  already  explained.  The  starting  point  /0  in  the  above  is  of 
course  the  temperature  at  which  the  cold  junctions  were  kept 
during  the  original  calibration,  often  o°  or  20°  C. 

Thus,  if  the  cold  junction  is  at  25°  C.  and  the  hot  at  500°,  this 
correction,  from  the  above  table,  is  +0.60  (25  —  o)  =  +15°,  if 
the  couple  was  calibrated  from  o°  C.,  and  the  galvanometer  read 
zero  for  a  cold-  junction  temperature  of  25°. 

It  is  a  simple  procedure,  and  usually  sufficiently  exact  when 
the  temperature  scale  of  the  galvanometer  corresponds  approxi- 
mately to  that  given  by  the  thermocouple,  to  set  the  pointer  of 
the  galvanometer  at  the  position  on  its  scale  corresponding  to 
the  temperature  of  the  cold  junctions.  The  readings  of  the  gal- 
vanometer, otherwise  corrected,  will  then  give  temperatures. 

Elimination  of  Cold-junction  Changes.  —  The  Bristol  base- 
metal  thermocouples  are  provided  with  extension  pieces  of  the 
same  composition  as  the  fire  end,  permitting  the  cold  junction  to 


THERMOELECTRIC  PYROMETER 


157 


be  removed  to  a  place  of  slight  temperature  change,  as  near  the 
floor,  and  this  arrangement  also  facilitates  the  convenient  renewal 
of  the  short,  heavy  fire  ends  of  these  couples  when  they  have  to 
be  discarded. 

Bristol  has  also  devised  an  automatic  compensator  for  cold-end 
temperatures,  shown  in  Fig.  43,  consisting  of  a  small  glass  bulb 
and  capillary  tube  partially  filled  with  mercury,  into  which 
a  short  loop  of  fine  platinum  wire  dips.  This  is  inserted  in  the 
thermoelectric  circuit  close  to  the  cold  junction,  Changes  in 


Thermo-electric 


Couple 


t 


Compensator 


Fig.  43.     Bristol's  Compensator. 


temperature  cause  the  mercury  to  expand  or  contract,  cutting 
in  or  out  resistance  in  the  circuit.  This  acts  in  opposition  to  the 
change  in  E.M.F.  with  temperature  at  the  cold  end,  so  that  a 
balance  may  be  established  if  the  parts  are  properly  designed. 

In  the  Thwing  instruments,  the  elimination  of  the  temperature 
variations  of  the  cold  ends  of  the  couple,  where  they  can  be 
brought  close  to  the  galvanometer,  is  affected  by  a  device  con- 
sisting of  a  compound  strip  of  two  metals  having  unequal  coeffi- 
cients of  expansion,  so  attached  to  the  spring  controlling  the 
pointer  that  the  reading  of  the  galvanometer  when  no  current  is 
flowing  is  the  temperature  of  the  surroundings. 


158 


HIGH  TEMPERATURES 


1  In  many  industrial  estab- 
lishments running  water  of 
practically  constant  tem- 
perature is  available,  and 
the  cold  end  of  the  thermo- 
couple can  then  be  water- 
jacketed  and  so  kept 
sufficiently  constant  in 
temperature,  as  shown  in 
Fig.  44,  which  represents 
an  arrangement  for  this 
purpose  as  constructed  by 
Hartmann  and  Braun. 
The  movable  arm  can  be 
swung  out  horizontally 
when  the  thermocouple  is 
to  be  immersed. 

Paul  provides  an  attach- 
ment by  which  an  inexpen- 
sive supplementary  couple, 
with  one  end  water-cooled, 
is  placed  in  series  with  and 
in  opposition  to  the  main 
thermocouple  by  means  of 
nonreversible  plugs  which 
fit  into  sockets  in  the  head 
of  the  pyrometer  cane. 
The  temperature  difference 
then  indicated  by  the  in- 
strument is  that  between 
the  fire  end  and  water- 
cooled  end. 

A     Breguet     spiral,    to 
which  one  end  of  the  con- 
trol   spring    of    the    milli- 
voltmeter  is  attached,  has  been  devised  by  C.  R.  Darling  and 


Fig.  44.     Water-jacketed  Cold  Junction. 


THERMOELECTRIC  PYROMETER 

sold  by  Paul.  In  this  way  the  zero  of  the  instrument  is  made 
to  vary  with  its  temperature. 

The  Crompton  Company  provide  their  instruments  with  a 
multiple  scale  (Fig.  45),  which  allows  for  the  cold-junction  tem- 
perature variations. 

Finally,  the  cold  end  of  the  thermocouple  may  be  buried  in  a 
box  underground,  for  instance,  and  copper  wires  run  to  the 
galvanometer.  We  shall  mention  other  such  devices  under  the 
heading  "  Compound  Thermocouples "  and  when  discussing 
accessories  to  recorders. 


Fig.  45.     The  Crompton  Scale. 

Constancy  of  Thermocouples.  —  This  matter  is  of  the  greatest 
importance  in  thermoelectric  measurements  both  in  the  labora- 
tory and  in  the  works,  as  there  is  nothing  more  aggravating  than 
the  gradual  deterioration  of  a  product  due  to  insidious,  and  often 
unnoticed  until  too  late,  changes  in  the  controlling  apparatus. 

The  behavior  of  thermocouples  made  of  platinum  and  its 
alloys  has  been  studied  in  great  detail,  from  this  point  of  view, 
by  several  observers,  but  the  data  are  somewhat  contradictory. 

If  a  thermocouple,  however  well  protected,  is  heated  for  a  long 
time  at  a  high  temperature,  its  E.M.F.  will  change.  It  is  well 
for  accurate  work  to  have  at  least  two  thermocouples,  one  of 
which  is  kept  as  a  standard  and  only  occasionally  heated,  and 
never  above  1200°  C.  In  this  way  changes  in  the  couple  ordi- 
narily used  may  be  readily  detected.  Holborn,  with  Henning 
and  Austin,  has  made  a  very  complete  study  of  the  effects  of 
continued  heating  in  various  atmospheres  on  the  loss  of  weight 


i6o 


HIGH  TEMPERATURES 


and  changes  produced  in  electric  and  thermoelectric  properties 
of  the  platinum  metals.  The  following  table  shows  the  results 
of  continued  heating  in  air  on  the  E.M.F.  of  the  platinum  couples 
ordinarily  used: 

EFFECT    OF   PROLONGED   HEATING   ON    E.M.F. 
E.M.F.    AGAINST   PT   IN   MICROVOLTS. 


Duration  of 
heating, 
hours. 

90  Pt  —  10  Ir. 

700°  C. 

900° 

1100° 

1300° 

O 

6 
8 

0 

i 

9 

12 

9460 
9160 
8840 

16,540 
15,450 
14,780 
14,300 

19,740 
18,530 
17,640 
17,050 

12,450 
H,930 
11,560 

90  Pt  —  10  Rh. 

800°  C. 

900° 

1000° 

1100° 

7230 
7250 
7270 
7280 
7290 

8340 
8380 
8400 
8410 
8420 

9480 
9510 

9540 
9540 
9550 

10,670 
10,690 

10,720 

This  investigation  shows  that  the  E.M.F.  of  a  couple,  and 
thus  the  indicated  temperature,  changes  with  continued  heating, 
very  considerably  for  a  Pt-Ir  couple  and  about  0.5  per  cent 
for  a  Pt-Rh  couple  for  ten  hours'  heating.  The  change  is 
greatest  during  the  first  part  of  the  heating.  The  observed  in- 
crease in  E.M.F.  of  the  Pt-Rh  is  difficult  to  explain  unless  it 
be  due  to  distillation  of  iridium  from  the  heating  coil,  as  shown 
from  Day  and  Sosman's  work.  Before  use,  a  thermocouple 
should  be  annealed  by  passing  a  current  through  it  at  a  white 
heat,  when  future  changes  will  be  slight  if  used  in  an  oxidiz- 
ing atmosphere.  This  annealing  also  will  restore  to  very  nearly 
its  normal  value  the  E.M.F.  of  couples  which  have  been  in 
contact  with  silicates. 

Changes  in  temperature  distribution  along  the  wire  may  also 
affect  the  apparent  electromotive  force  of  the  couple,  causing 


THERMOELECTRIC   PYROMETER  l6l 

apparent  changes  in  temperature  as  great  as  20°  at  1000°  C.  with 
some  wires.  The  less  homogeneous  the  wires  the  more  marked 
is  this  effect.  In  the  most  exact  work,  therefore,  the  same  con- 
ditions of  immersion  must  be  followed  throughout,  or  the  result- 
ing changes  in  E.M.F.  measured. 

It  follows  from  all  this,  as  Holborn  and  Day  state,  that  the 
temperature  scale,  once  established  by  means  of  the  thermo- 
couple, can  be  maintained  with  certainty  only  with  the  help  of 
fixed  temperatures  such  as  the  melting  points.  Dr.  W.  P.  White 
draws  particular  attention  to  the  importance  of  that  region  of 
the  wires  passing  through  a  steep  temperature  gradient  and 
the  great  influence  that  inhomogeneity  in  this  portion  of  the 
wire  may  have  upon  the  temperature  readings  with  thermo- 
couples. 

If  we  consider  an  inhomogeneous  thermocouple  composed  of 
short  segments  each  of  which  is  supposed  homogeneous,  at  any 
junction  of  two  segments  there  is  developed  an  E.M.F.  pro- 
portional to  the  temperature  t  and  their  difference  in  thermo- 
electric power  A/7,  or  for  the  whole  circuit: 

E  =  (fc-AFi  +  fe  -A#2  +   .  •  •  /n-A£Tn)  =  S*-A#. 

It  is  evident  that  those  portions  of  the  circuit  at  constant  tem- 
perature and  of  homogeneous  material  (&H  =  o)  do  not  con- 
tribute to  the  value  of  E;  but  in  the  regions  of  temperature 
gradient  of  an  inhomogeneous  wire,  the  errors  due  to  inhomo- 
geneity depend  also  upon  the  temperature  distribution  along 
the  wire.  If  an  inhomogeneous  thermocouple,  therefore,  is 
raised  or  lowered  in  a  furnace  at  constant  temperature,  the 
reading  of  the  couple  will  change.  In  view  of  these  facts,  it  is 
important  that  those  portions  of  the  thermocouple  wires  pass- 
ing from  cold  to  hot  regions  be  chemically  and  physically  of 
uniform  properties. 

The  effect  of  initial  chemical  inhomogeneity,  for  the  platinum 
thermocouples,  appears  to  be  either  negligible  or  very  small,  but 
may  be  considerable  for  base-metal  couples.  The  region  between 
hard-drawn  and  annealed  wire  is  one  of  marked  physical  inhomo- 


162  HIGH   TEMPERATURES 

geneity.  This  can  .be,  and  should  always  be,  eliminated  by 
annealing  the  wires  of  most  couples  including  platinum,  pref- 
erably with  an  electric  current,  or  by  hardening  in  the  case  of 
constantan.  The  most  troublesome  source  of  inhomogeneity, 
however,  the  most  difficult  to  remove,  and  the  source  of  greatest 
error,  particularly  with  platinum  thermocouples,  is  that  due  to 
contamination  from  evaporation  and  diffusion  of  metal  vapors 
into  the  region  of  temperature  gradient  of  the  platinum  wire. 
The  oxide  coating  which  forms  on  some  metals  is  also  a  similar 
source  of  uncertainty  in  the  regions  of  variable  temperature. 

Regarding  the  effects  of  contamination  of  platinum  wires,, 
carbon,  illuminating  gas,  and  other  reducing  agents  appear  to 
act  only  through  their  reducing  action  on  other  substances,  such 
as  iron  and  silicon,  capable  of  injuring  the  platinum.  In  an 
oxidizing  atmosphere,  iron  oxides  and  silicates  produce  little  or 
no  effect;  but  metals  such  as  iridium  and  rhodium,  particularly 
the  former,  which  is  very  volatile  even  from  a  Pt  -  Ir  alloy  wire 
at  900°  C.,  and  capable  of  alloying  with  platinum,  will,  if  present,, 
produce  marked  contamination  of  platinum  wires.  Amputation 
of  the  contaminated  portions  appears  to  be  the  only  remedy  in 
this  case.  Excessive  local  heating  will  also  cause  inhomogeneity 
to  develop,  especially  in  the  alloy  wire,  probably  due  in  part  to 
evaporation  and  in  part  to  crystallization. 

Measurement  of  Inhomogeneity.  —  This  is  very  easily  and  ex- 
actly effected,  and  should  be  carried  out  on  any  thermocouples 
to  be  used  in  work  of  high  accuracy.  Each  of  the  wires  is  tested 
separately,  its  ends  being  kept  conveniently  at  a  constant  tem- 
perature of  o°.  The  wire  is  in  circuit  with  a  sensitive  galva- 
nometer graduated  in  microvolts  and  is  passed  through  a  short 
electric  resistance  furnace  kept  at  a  constant  temperature,  1000° 
or  1400°  C.  The  readings  of  the  galvanometer  are  taken  for 
different  positions  of  the  furnace  along  the  wire.  The  furnace 
may  be  replaced  conveniently  by  a  short  length  of  porcelain 
heated  by  a  Bunsen  burner.  Using  a  sensitive  recorder,  a  small 
furnace  may  be  pulled  automatically  along  the  wire  which  records 
its  own  variations.  Such  a  device  is  in  use  at  the  Bureau  of 


THERMOELECTRIC   PYROMETER 


163 


at  •  j  'H  '3 


164  HIGH  TEMPERATURES 

Standards.  Another  method  is  that  shown  in  Fig.  47,  which 
permits  a  point-to-point  study  of  the  phenomenon.  In  Fig.  46 

are  shown  the  homogeneity 
curves,  taken  by  the  first 
method,  of  the  wires  of  two 
thermocouples,  the  one  (Pt- 

Examination  . 

Rh)  new  and  of  fresh,  pure 

Fig.  47-    Testing  of  Homogeneity.        materialS)    ^    other    (pt_jr) 

old  and  impure.  These  methods  are  easily  sensitive  enough  to 
differentiate  the  various  grades  of  platinum  wire  used  in  thermo- 
couple manufacture. 

We  have  already  called  attention  to  the  inhomogeneity  of  base- 
metal  thermocouples.  We  shall  return  to  this  question  in  a  later 
paragraph. 

Reproducibility  of  Thermoelectric  Apparatus.  —  It  is  often  of 
considerable  convenience  in  all  kinds  of  measurements,  especially 
on  a  large  scale  with  numerous  working  units  of  the  same  kind, 
to  be  able  to  duplicate  or  replace  corresponding  parts  without 
having  resort  to  new  calibrations.  This  is  equally  desirable  in 
temperature  measurements,  and  in  recent  years  there  has  been 
a  large  measure  of  success  in  the  attempt  to  produce  thermo- 
couples and  manufacture  pyrometer  galvanometers  which  are 
interchangeable  . 

As  an  example  of  the  former,  we  may  cite  the  case  of  the  well- 
known  10  per  cent  platinum-rhodium  normal  thermocouples  of 
Heraeus.  They  are  reported  to  have  maintained  the  following 
constancy  for  the  past  six  years: 

AVERAGE  E.M.F.   AT   1000°  C. 

.  Year.  Millivolts. 

1904  9-52 


9-53 
1906       9  .  53 

J907       9-57 
1908       9.59 

9-55 


Regarding  the  interchangeability  of  pyrometer  galvanometers, 
the  art  of  electrical  instrument  manufacture  has  so  advanced  in 


THERMOELECTRIC  PYROMETER  165 

recent  years  that  equivalent  instruments,  in  error  absolutely  and 
with  respect  to  each  other  by  less  than  10°  C.  throughout  their 
scales,  are  produced  currently  by  several  makers. 

Base-metal  Thermocouples.  —  There  appears  to  be  an  insistent 
demand,  on  the  part  of  many  in  charge  of  technical  processes 
requiring  temperature  control,  for  inexpensive  and  robust  measur- 
ing apparatus.  For  this  reason,  if  for  no  other,  the  use  of  the 
base-metal  thermocouple  has  become  firmly  established.  Its 
success  has  been  due  to  several  causes,  principal  among  them 
being  the  production  of  fairly  satisfactory  alloys  of  high  E.M.F. 
with  temperature  change,  which  can  be  made  into  practically 
unbreakable  pyrometric  canes  of  very  low  resistance;  and  the 
simultaneous  development  of  pivot  millivoltmeters  suitable  for 
use  as  galvanometers  with  this  type  of  couple. 


Fig.  48.     Heavy  Base-metal  Welds. 

Such  canes  can  be  had,  for  example,  with  a  resistance  as  low 
as  0.05  ohms  cold,  increasing  by  only  o.oi  ohm  when  heated  to 
1000°  C.  Even  with  a  commercial  milli voltmeter  of  only  i  ohm 
resistance,  the  calibration  will  remain  constant,  under  these 
conditions,  to  10°  C.  for  any  depth  of  immersion  of  the  couple 
(see  page  1 20) .  It  is  indispensable  in  such  cases  that  all  junctions 
be  of  negligible  resistance,  and  they  are  preferably  soldered.  In 
Fig.  48  are  illustrated  two  types  of  weld  for  base-metal  couples. 

A  very  considerable  error,  however,  in  the  estimate  of  the 
temperature  of  regions  into  which  are  thrust  thermocouples  of 
considerable  cross  section  and  insufficient  length,  may  arise  from 
heat  conduction  along  the  pyrometer,  the  effect  being  to  chill  the 


l66  HIGH  TEMPERATURES 

hot  junction  below  the  temperature  of  its  surroundings.  It  is 
important  that  such  pyrometers  be  calibrated  to  allow  for  this 
effect. 

The  thermoelectric  power  of  many  of  these  base-metal  couples 
is  over  20  microvolts  per  i°  C.,  and  some  of  them  are  40  or  more 
as  compared  with  10  microvolts  per  i°  C.  for  the  ordinary  plati- 
num-rhodium couple.  The  low-resistance  pivot  galvanometers 
of  20  or  40  millivolts  range,  suitable  for  use  with  the  base-metal 
couples,  may  be  made  more  cheaply  and  robust  than  instruments 
suitable  for  use  with  platinum  couples  for  the  same  sensibility. 

As  the  base-metal  couples  receive  hard  usage,  are  cheap,  and 
often  require  frequent  replacing,  it  is  of  great  advantage  to  make 
use  of  material  of  uniform  thermoelectric  properties,  so  that 
burned-out  "  fire  ends  "  may  be  readily  replaced  as  required 
without  retesting.  It  is  of  course  safer,  and  necessary  in  many 
instances,  to  calibrate  each  new  fire  end,  either  by  comparison 
with  a  standard,  or,  as  may  be  done  conveniently  even  in  tech- 
nical plants,  by  taking  the  reading  in  salt  baths  of  known  freezing 
points  (see  page  190).  In  the  case  of  the  use  of  alloys  or  metals 
possessing  critical  regions,  accompanied  by  the  absorption  or 
liberation  of  heat,  it  should  be  emphasized  that  discordant  results 
may  be  obtained  on  reheating,  the  actual  E.M.F.- temperature 
relation  depending  upon  the  internal  structure  of  the  material 
of  the  couple,  and  this  in  turn  upon  the  rate  of  heating  or  cooling 
through  these  critical  regions.  These  effects  are  particularly 
marked  in  couples  of  considerable  size,  and  are  enhanced  by 
varying  the  depth  of  immersion  of  the  couple  in  the  test  bath 
or  furnace.  We  shall  mention  later  some  specific  instances  of 
these  effects. 

Although  a  very  considerable  number  of  base-metal  thermo- 
couples have  been  put  upon  the  market  in  recent  years,  there 
appears  to  be  very  little  certain  knowledge  available  as  to  the 
exact  composition,  thermoelectric  properties,  and  behavior  of 
most  of  them,  some  of  which  are  quite  complex  alloys,  as  for 
example  of  Ni,  Cr,  Al,  and  Cu.  It  should  perhaps  be  noted  that 
our  use  with  base-metal  couples  of  the  adjectives  "  constant," 


THERMOELECTRIC  PYROMETER  167 

"  reproducible/'  etc.,  is  not  to  be  taken  in  the  same  rigorous 
sense  as  for  the  platinum  couples. 

Nickel-copper.  —  Various  combinations  of  these  metals  have 
been  used  and  recommended  for  temperatures  as  high  as  900°  C. 
According  to  the  investigations  of  M.  Pecheux,  the  most  satis- 
factory one  seems  to  be  that  with  the  alloy  constantan  (known 
also  as  "advance"),  60  Cu— 40  Ni,  as  one  wire  and  pure  Cu 
as  the  other.  The  E.M.F.-temperature  curve  for  this  thermo- 
couple approximates  a  fairly  flat  parabola,  but  appears  to  require 
an  equation  of  the  third  or  fourth  degree  in  /  to  express  the  results 
with  some  exactness  to  900°  C.;  or  this  interval  may  be  divided 
into  three,  each  of  which  is  represented,  to  a  fraction  of  a  degree, 
by  a  parabola  of  the  form  E%  =  a  +  bt  +  ct2.  Between  o  and 
250°  C.  the  numerical  equation  is  roughly  E*Q  =  40  /  +  0.03 /2 
in  microvolts  for  the  copper-constantan  thermocouple.  Those 
with  a  smaller  percentage  of  nickel  have  less  flat  curves,  lower 
thermoelectric  powers,  and  guard  their  original  calibration  less 
well  than  does  the  copper-constantan  couple. 

The  limiting  case  in  this  series,  that  of  pure  nickel  against  pure 
copper,  is  of  some  interest,  as  it  gives  a  good  illustration  of  the 
effects  of  molecular  transformation  on  thermoelectric  behavior. 
Nickel  undergoes  such  transformation  between  about  230°  and 
390°  C.,  which  causes  both  its  electrical  resistance  and  thermo- 
electric power  to  depart  from  their  normal  trend  in  this  region. 
These  effects  are  shown  in  Fig.  49,  the  data  on  resistance  being 
from  some  measurements  made  by  Somerville,  on  nickel  wire,  and 
on  thermoelectric  power  of  the  Ni-Cu  couple  from  the  observa- 
tions of  M.  Pecheux. 

When,  for  such  a  couple,  or  any  thermocouple  in  which  nickel 
or  any  substance  possessing  regions  of  molecular  transformation, 
its  rate  of  heating  or  cooling  is  varied,  the  E.M.F.  readings  of 
the  couple  will  not  in  general  be  the  same  for  a  given  temperature 
within  this  region;  and  for  rapid  cooling,  in  some  cases,  especially 
for  wires  or  rods  of  considerable  diameter,  the  E.M.F.-tempera- 
ture relations  may  be  changed  at  all  temperatures  below  this 
region  as  well,  due  to  the  retardation  or  partial  prevention  of 


i68 


HIGH  TEMPERATURES 


the  complete  transformation  by  chilling.  Reannealing  and  slow 
cooling  will  oftentimes  restore  the  original  annealed  condition. 
The  importance  of  annealing  all  such  couples  before  their  first 
calibration  becomes  apparent  from  the  above  considerations. 

The  presence  of  impurities  appears  to  be  a  further  source  of 
considerable  uncertainty  in  the  constancy  of  these  couples  with 
continued  use,  it  being  noted,  by  Pecheux  for  example,  that 
couples  with  very  pure  nickel  remained  more  constant  in  use 
than  those  with  the  less  pure  metal. 


NICKEL 

RESISTANCE    -H--f 
THERMOELECTRIC  POWER  AGAINST  COPPER   o  o  o 


500 


420 


340. 


260- 


180 


100 


100°       300°       300°       400°       500°       600°       700°       800°       900°      1000° 
Temperature  Centigrade 

Fig.  49.     Resistance  and  Thermoelectricity  of  Nickel. 

The  addition  of  zinc  in  any  proportions  to  the  copper-nickel 
alloy,  giving  the  German  silvers,  appears  to  be  detrimental  in 
all  respects. 

The  upper  limit  of  900°,  assigned  by  Pecheux  for  the  copper- 
constantan  couples,  is  better  replaced  by  600°,  or  even  less,  for 
continued  use  with  any  considerable  precision ;  and  even  at  600° 
both  wires  will  oxidize  and  soon  become  fragile. 

Nickel-iron.  —  We  have  already  called  attention  to  the  be- 
havior of  iron  wires  and  the  enormous  parasite  E.M.F.'s  that 
they  may  develop.  Nevertheless,  a  favorite  industrial  thermo- 
electric combination  has  been,  and  still  is  in  some  quarters,  a 


THERMOELECTRIC   PYROMETER  169 

tube  of  soft  iron  inclosing  a  nickel  wire.  Nickel,  associated 
with  any  other  metal  or  alloy,  will  furnish  a  couple  possessing 

anomalies  in  -  -  below  400°  C.,  due  to  its  molecular  transforma- 
at 

tion  region.  Similarly,  iron  and  also  the  various  carbon-  and 
alloy-steel  wires  have  been  found  to  introduce  erratic  thermo- 
electric behavior  when  used  as  one  element  in  thermocouples. 

Harrison,  Barrett,  Belloc,  and  others,  who  have  studied  in 
detail  the  behavior  of  such  thermocouples,  find,  for  example,  that 
there  are  considerable  changes  in  E.M.F.'s  due  to  oxidation, 
carburization,  reheating,  rate  of  cooling,  the  nature  of  the  furnace 
atmosphere,  maximum  temperature  reached,  and  time  of  heating 
at  any  temperature;  and  in  general,  within  any  region  of  molec- 
ular transformation,  the  E.M.F.  on  cooling  will  differ  from  that 
on  heating,  producing  E.M.F. -temperature  cycles  or  hysteresis. 
The  couples  containing  iron  are  most  unreliable  above  800°  C., 
and  all  such  couples,  as  well  as  those  with  wires  of  steel, 
nickel,  copper,  and  many  of  their  alloys,  become  brittle  above 
700°  C. 

The  effect  of  reheating  in  producing  hysteresis  is  shown  in  an 
experiment  of  Barrett  on  the  change  in  temperature  of  the  neu- 
tral point  of  a  copper-steel  couple : 

NEUTRAL   POINT  OF   COPPER- STEEL. 


— Heating.  — 


Second.  Third. 


When  heating 328  283  268 

When  cooling 258  241  241 

The  E.M.F.  is  not  always  higher  during  heating,  however, 
the  following  alloy  for  example  giving  lower  E.M.F.'s  against 
Cu,  Pt,  or  Fe  on  heating.  This  alloy  (Fe  =  68.8,  Ni  =  25.0, 
Mn  =  5.0,  C  =  1.2),  due  to  Sir  Robert  Hadfield,  also  pos- 
sesses the  remarkable  property  of  giving  against  a  nearly  pure 
iron  an  E.M.F.  constant  to  within  4  per  cent  from  300°  to 
1050°  C. 

The  variation  in  electromotive  force  with  composition  of 
steels  against  platinum  has  been  studied  by  Belloc,  whose  results 


170  HIGH  TEMPERATURES 

are  shown  in  a  somewhat  idealized  form  in  Fig.  23,  page  no, 
from  which  it  is  evident  that  any  changes  in  composition  due 
to  heating  or  atmosphere  will  produce  great  changes  in  the 
E.M.F.  of  such  couples  below  350°  C.  and  above  700°  C.  By 
heating  a  platinum,  1.2  per  cent  carbon-steel  couple  fifteen 
times  to  1000°,  its  E.M.F.  per  degree  at  800°  changed  from  n 
to  19  microvolts. 

On  the  other  hand,  for  relatively  low  temperatures,  very  exact 
measurements  have  been  made  with  thermocouples  made  of  iron 
or  nickel  and  a  base-metal  alloy.  Thus  Palmer,  working  in  the 
range  o°  to  200°  C.  with  an  iron-cons  tan  tan  couple,  gets  a  pre- 
cision of  0.04  per  cent  when  the  residual  E.M.F.'s,  including 
those  due  to  mechanical  strains,  are  eliminated. 

Complex  Alloy  Couples.  —  Several  manufacturers  have  sought 
to  produce  base-metal  couples  which  are  free  from  some  of  the 
defects  usually  present  in  this  type.  The  work  done  so  far  gives 
promise  that  this  field  of  investigation  is  worthy  of  further  de- 
velopment. Most  of  the  effort  has  been  on  the  modification  of 
the  iron  and  nickel  elements  by  the  addition  of  other  metallic 
components  such  as  tungsten,  copper,  chromium,  cobalt,  silicon, 
and  aluminium.  We  may  mention  the  Bristol,  Thwing,  and  Hos- 
kins  couples  as  examples.  One  of  the  last,  a  robust  nickel-chro- 
mium combination  (Ni,  90  Ni  — 10  Cr),  has  an  E.M.F.  about 
four  times  that  of  the  ordinary  Pt-Rh  couples,  the  E.M.F. - 
temperature  relation  is  nearly  linear  to  1400°  C.  without  any 
recalescence  disturbances  of  sufficient  magnitude  to  seriously 
affect  the  temperature  readings  in  technical  work,  and  after  an- 
nealing this  couple  retains  its  readings  sufficiently  for  many 
commercial  uses,  even  when  heated  to  over  1300°  C.  for  short 
periods. 

For  high  temperatures,  the  Hoskins  Company  now  use  prin- 
cipally the  couple  nickel  aluminium  (2%  Al)— nickel  chromium 
(10%  Cr);  and  for  low  temperatures,  nickel  copper  (65%  Cu) 
—nickel  chromium  (10%  Cr).  The  characteristics  of  the  mate- 
rials used  in  the  Hoskins  couples  are  given  by  M.  A.  L.  Marsh, 
as  shown  in  the  accompanying  table : 


THERMOELECTRIC  PYROMETER  171 

HOSKINS'  THERMOCOUPLE  CALIBRATIONS. 
COLD  JUNCTION  AT  25°  C. 

Couple.  Millivolts. 

Positive  element.          Negative  element.  100°  C.  232°  C.   419°  C.  657°  C.   800°  C.  1065°  C. 

Nickel Nickel  chromium,  10%  Cr.  3.21       9.34      16.88  26.0  31.50        41.80 

Nickel  aluminium, 

2%A1 Nickel  chromium,  10%  Cr 9.35      17.60  28.10  34.53        46.20 

Nickel  aluminium, 

3$%A1 Nickel  chromium,  10%  Cr.      8.75      17.20  28.25  35.00         47.20 

Nickel  aluminium, 

5%  Al Nickel  chromium,  10%  Cr.      8.25      16.67  47.00 

Cobalt Nickel  chromium,  10%  Cr.  3.09        9-93      20.06  33. 18  45.58 

Nickel  copper,  , 

65%  Cu Nickel  chromium,  10%  Cr.  4.39      13.15      27.27  44.66  75-40 

Copper Nickel  chromium,  10%  Cr.  1.48        4.15        7.73  11.27  13-57 


ELECTRICAL   RESISTANCE   OF  THERMOCOUPLE   ELEMENTS. 

Resistance  per  foot,  qnpHfir  _  Temperature 

Element.                            0.40  m/m.  wire,  &pecihc  resistance  coefficient 

at  25°  C.  per  degree  C. 

Nickel 0.26  ohm                  10.34  0.00415 

Nickel  aluminium,  3!%  Al.  .          .63                           25.0  .00274 

Cobalt 356  14-15  

Nickel  copper,  65%  Cu 99  39.3  

Copper 043                           1.75  . 00388 

Nickel  chromium,  10%  Cr. . .       i .  76                           70.0  .00051 

A  difficulty  met  with  in  the  manufacture  of  complex  alloy 
couples  is  the  reproducibility  of  the  same  E.M.F. -temperature 
relation  from  one  casting  to  another.  The  identity  of  behavior 
is,  however,  highly  desirable  in  cheap  commercial  couples  which 
are  frequently  replaced,  as  it  obviates  the  necessity  of  recalibra- 
tion  or  adjustment  of  the  galvanometer  scale  for  each  couple. 

The  Noble  Metals:  Geibel's  Data. —  We  have  already  dis- 
cussed the  thermoelectric  behavior  of  the  platinum-rhodium 
alloys,  page  116.  Some  of  the  platinum  group  metals  and  alloys 
have  been  studied  by  Holborn  and  Day,  Rudolphi,  Doerinckel, 
and  others.  The  most  thorough  and  reliable  investigation, 
however,  of  the  electrical  and  mechanical  properties  of  the  noble 
metals  and  their  alloys,  in  view  of  their  availability  for  tempera- 
ture measurement,  has  been  made  by  W.  Geibel  in  the  laboratory 
of  the  Heraeus  platinum  works,  using  materials  much  purer  than 
could  be  obtained  by  Barus  or  Le*  Chatelier  twenty-five  years 
ago.  The  data  of  Geibel  show  wide  differences  from  the  earlier 
results  of  these  observers. 


172 


HIGH  TEMPERATURES 

PROPERTIES  OF  THE  NOBLE 


Electromotive  force  (millivolts)  against 

Metal  or  alloy. 

100° 

200° 

300° 

400° 

500° 

600° 

700° 

800° 

900° 

Pd  I 

—     I.I 

-   i.  9 
—  4.1 
-   7.0 
—   i  .0 
+  1.8 

—    2.0 

-  3-5 
-  6.8 

—    O.2 

+   1.7 
+  0.6 
+   1.6 
+  i.S 
+  0.7 

—    i.i 

-   1.8 
-   2.9 
-  6.4 
-10.5 
-   1.7 
+  3-1 
-   3-3 
-   5-7 
-10.7 

+  V.9 
+  0.8 
+  2.5 
+  2.3 

+    1.2 

-    1.6 
-   2.6 
-  4.1 
-  9.4 
-15-0 
-   2.5 
+  4-5 
-   5-o 
-  8.5 
-15-3 
-  0.3 
+  4-4 

+    I.O 

+  3-5 
+  3-3 
+   i.7 

—     2.4 

-   3-4 
-    5-4 
-12.5 
-19.7 
~   3-4 

+    6.2 

-  6.8 
-II.  7 

—  20.2 

+  Y.I 
+    I.O 

+  4-4 
+  4-4 
+   2.3 

-  3-3 
-  4-6 
-  6.9 
—  16.0 

-24.2 

-  4-4 
+  8.0 
-  9.0 
-15.2 

-25-4 
-  0.4 

+    8.2 

+  0.8 
+  5-3 

+  5-4 
+  2.7 

-  4-5 
-  6.0 
-   8.4 
-19.7 
-28.8 

+  9-9 
-ii.  4 
-19.2 
-31-0 

+  10.6 
+  0.6 

+    6.2 

+  6.5 

+  3-2 

-   5-8 
-   7-5 

—  IO.2 
-23-4 

-33-5 
-  6.8 

+  12.  0 
—  14.2 
-23.2 

-36.5 
-    0.5 

+  I3-I 
+    O.I 

+  6.8 
+   7-6 
+  3-7 

-   7-3 
-  9.2 
—  12.  i 

-27.3 
-38-2 
-   8.2 

+14.2 

-17.0 
-27.4 
-42.1 

-  0.6 

+  15-9 
-  0.6 

+  7-4 
+  8.5 
+  4-2 

+"9.7 

+  14.0 

+17.1 

+  16.8 
+  10.3 
+  n.  7 

>lts  at 

4-7 
4-5 
6.6 
6.8 

Pd  II 

-3'-8' 
-0.5 
+0.8 
-0.9 
-1.8 
-3-7 

—  O.I 

+0.7 
+0.3 
+0.8 
+0.7 
+0-3 

Pd-Au  10.  .  . 
Pd—  Au40.  .  . 
^  Pd-Au  60.  .. 
Pd-Au  80... 
Au 

Pd—  Ag  10.  .  . 
Pd—  Ag  20.  .  . 
Pd-Ag4o.  .. 
Pd-Ag8o.  .. 
Ag 

Pd-Ptio.... 
Pd-Pt3o.... 
Pd-Pt6o.... 
Pd-Ptpo.... 
Pt 

Pt-Ir    5.... 
Pt-Ir  10  
Pt-Ir  25.... 
Pt-Ir  35.... 
Ir* 
Rh* 

+  1.1 
+  1-3 
+  1.2 
+  1.1 
+0.65 
+0.65 

! 

+0.2 
-0.4 

+  2.1 

+    2.6 
+     2.6 

+  2.5 
+  i-S 
+   i-S 

Not  cc 

+  0.4 
-  0.8 

+    3-2 

+  4-1 
+  4-3 

+  4-1 
+   2.5 

+    2.6 

>nstant 

+  0.7 
-   1.4 

+  4-3 
+  5-8 

+    6.2 

+  5-9 
+  3-6 
+  3-7 

varia 
!    1.3 

1      I.O 

t.o 

(      2.1 

+  5-4 
+  7-4 

+    8.2 

+  7-9 
+  4-8 
+  5-1 

tions  a 

1.8 
i-5 

2.6 
2.8 

+  6.5 
+  9-1 
+  10.4 

+  9-9 
+  6.1 

+  6.5 
s  grea 

2.4 

2.1 

3-5 

3-7 

+  7-6 
+  10.7 

+  12.6 
+  12.  1 

+  7-6 
+  8.1 

/  as  2 
3-i 

2.8 

4-5 
4-7 

+  8.7 
+  12.3 
+  14-8 
+  14-4 
+  9-1 
+  9-9 

millivc 

3-8 
3-6 
5-5 
5-7 

Au-Ptio.... 

AU—  Pt20.... 

Au—  Pt4o.... 
Ag-Ptio.... 

Ag-Pt3o.... 

*  Holborn  and  Day. 

Some  of  his  results,  taken  from  a  very  complete  series,  on 
E.M.F.  against  platinum,  electrical  conductivity,  temperature 
coefficient,  and  tensile  strength,  are  given  in  the  accompanying 
table.  Wires  of  1.3  mm.  were  first  glowed  and  then  cold-drawn 
to  i  mm.  Compositions  are  per  cent  by  weight.  The  tensile 
strength  may  be  taken  as  giving  an  approximate  measure  of 
hardness. 

One  of  the  most  satisfactory  combinations  for  use  as  thermo- 
couple to  say  1000°  C.  appears  to  be  40  Pd  •  60  Au  —  Pt,  which 
.at  1000°  C.  gives  four  times  the  E.M.F.  of  the  ordinary  Le  Chate- 
lier  couple.  This  Pd  •  Au  alloy  also  has  a  very  low  temperature 


THERMOELECTRIC  PYROMETER 

METALS  AND   THEIR  ALLOYS.     (GEIBEL.) 


173 


platinum. 

Electrical 
conductivity 
Xio-<ato°C. 

Temperature 
coefficient 
between  o° 
and  160°. 

Tensile  strength 
in  kg.  for 
i  mm.  wire. 

Melting 
begins.    From 
.  various 
observers. 

1000° 

1100° 

1200° 

-    8.Q 
—  II.  0 

-14.0 
-30.9 
-42-7 
-   9-8 
+  16.5 

-13.0 

-15.0 

9-47 

0.00328 

30 

1550 

7.01 
3.96 
4-05 
7-94 
47-52 
4-85 
3-26 
2.38 
9.58 
63.72 

6-93 
4-57 
3-78 
5.38 
9-94 
5-6i 
4-34 
3-17 
2.71 

.00224 
.00079 
.00034 
.00064 
.00326 
.00117 
.00066 
.00005 
.00047 
.0041 
.00214 
.00128 
.00096 
.00136 
.00348 
.00188 
.00126 
.00066 
.00058 

36 
43 
49 
45 
21-5 
42 
49-5 
Si 
40 

3i 

(3^) 
(43) 
42 
24 
40 
48 
98 

!26 

1545 
1500 

1450 
1350 
1063 
1500 
1450 

1350 
IIIO 

960 
1570? 



P 

+  9-5 

+  4-7 

-  2.5 

+  8.1 
H-io.6 

+    S-2 

;  i:i 

+  n.  5 

+  5.7 

1730? 

1755 
1780? 

+  10.7 
+  15-7 
+  19-4 
+  19.1 

+  12.6 

+13.7 

high  tei 

Before 
After  I 
Before 
After  \ 

+  11.  8 
+  I7-3 

+  21.8 
+  21.6 

+14.5 
+15.8 

nperatui 

heating 
leating 

+  19.0 

+24.3 

+24.3 

2300 
1920 
1630 

1510 

1340 
1450? 

res.      < 

9.76 

5-57 
3-o6 

.00098 
.00054 
.00037 

32 
52 
69 

heating 
leating  . 

1200? 

coefficient  and  resembles  the  10  per  cent  indium  alloy  of  platinum 
in  hardness. 

The  alloys  of  platinum  with  gold  or  silver  are  evidently  un- 
suitable for  use  in  thermocouples,  due  to  great  changes  in  E.M.F. 
with  prolonged  heating.  Geibel  gives  also  data  showing  the 
effect  of  annealing  at  various  temperatures  upon  tensile  strength 
for  some  of  these  alloys.  The  effect  is  most  marked  for  those 
alloys  which  show  corresponding  changes  in  E.M.F.  For  the 
Pt-Ir  alloys  there  is  little  effect  of  annealing  until  600°  is 
reached,  but  after  annealing  above  800°  the  tensile  strength 
(cold)  falls  off  rapidly  with  increase  in  temperature.  For  pure 


174  HIGH  TEMPERATURES 

platinum  this  decreases  regularly  from  32  kg.  for  the  hard-drawn 
to  17  kg.  after  annealing  at  1300°  C. 

The  alloy  60  Pd  •  40  Ag  has  nearly  a  zero  temperature  co- 
efficient. If  of  sufficient  permanence  in  its  properties,  when 
joined  to  90  Pt  •  10  Ir,  for  example,  we  should  have  a  couple  of 
nearly  constant  resistance  seven  times  as  sensitive  as  90  Pt  •  10  Rh 
-  Pt  at  900°  C. 

All  the  alloys  noted  in  the  table  appear  to  be  solid  solutions 
with  no  transformation  or  critical  points. 

Special  Couples.  —  We  may  mention  certain  couples  that  can 
be  classed  neither  as  base-metal  nor  as  platinum  thermocouples, 
some  suitable  for  relatively  low  temperatures  and  others  for  the 
very  highest. 

Silver -constantan  is  a  combination  that  is  used  considerably 
and  appears  to  give  satisfaction  to  temperatures  as  high  as 
700°  C. 

Silver-nickel  has  also  been  used  by  Hevesy  and  Wolff  from 
—  80°  to  920°  C.  The  thermoelectric  power  is  about  three  times 
that  of  the  Pt-Rh  couple,  but  is  quite  variable,  and  there  is 
no  simple  formula  expressing  the  E.M.F. -temperature  relation 
even  above  400°  C. 

Iridium-ruthenium.  —  The  upper  limit  for  the  continued  use 
without  frequent  recalibration  of  the  platinum-rhodium  couple 
is  about  1600°  C.,  although  the  melting  point  of  platinum  may 
be  reached  with  it.  Heraeus  has  met  the  need  for  a  couple  that 
can  be  used  to  very  much  higher  temperatures  by  constructing 
one  having  for  one  lead  pure  iridium  and  for  the  other  an  alloy 
of  90  parts  iridium  to  10  parts  ruthenium,  with  which  tempera- 
tures to  about  2100°  C.  may  be  measured.  The  E.M.F.-tem- 
perature  relation  for  these  couples  is  not  quite  linear. 

Calibrations  may  be  made  in  a  suitable  furnace  by  compari- 
son with  a  Pt-Rh  couple  or  by  taking  readings  at  the  melting 
points  of  Au,  Pd,  and  Pt,  above  which  temperature  extrap- 
olation must  be  resorted  to  unless  pure  Rh  is  available  as 
a  calibration  point,  or  comparison  be  made  with  an  optical 
pyrometer. 


THERMOELECTRIC  PYROMETER  175 

The  indications  of  this  couple  remain  fairly  constant  with 
repeated  heatings  considering  the  extremely  high  tempera- 
tures to  which  it  may  be  exposed.  Inhomogeneity  will  of 
course  develop.  A  serious  source  of  error,  noninherent  in  this 
couple  alone,  is  that  due  to  heat  conduction  along  the  thick 
leads,  which  may  amount  to  50  degrees  at  the  higher  tem- 
peratures unless  allowed  for  by  observing  some  known  tem- 
perature, as  the  Pt  point,  with  the  same  immersion  as  used  in 
the  experiments. 

This  thermocouple,  on  account  of  its  very  great  fragility 
when  cold,  is  not  suited  for  any  ordinary  industrial  uses,  and 
must  be  handled  with  the  greatest  care;  it  is  also,  of  course,  very 
expensive. 

Compound  Thermocouples.  —  These  are  of  two  kinds,  the 
object  of  the  first  being  to  give  greater  sensibility  to  the  couple 
by  increasing  its  E.M.F.  This  is  usually  accomplished  by  put- 
ting two  or  more  thermocouples  in  series,  when  the  available 
E.M.F.  is  increased  in  the  proportion  to  the  number  of  couples. 
It  should  be  remembered,  however,  that,  by  this  operation,  the 
electrical  resistance  of  the  circuit  is  also  proportionally  increased, 
and  this  may  introduce  considerable  errors  when  indicating  gal- 
vanometers of  relatively  low  resistance  are  used;  and  in  this  case 
also,  the  sensibility  may  not  be  increased  enough  to  warrant  the 
additional  couples,  which  are,  of  course,  expensive  if  the  platinum 
metals  are  used.  The  effects  of  varying  depths  of  immersion 
of  the  couple  wires  in  the  heated  space  and  changes  of  zero  are 
also  accentuated  by  this  method,  which,  with  the  recent  devel- 
opment of  galvanometers  which  are  both  sensitive  and  robust, 
becomes  superfluous  in  ordinary  cases.  For  measuring  small 
temperature  differences,  however,  as  in  detecting  transformation 
points,  this  method  has  its  advantages. 

The  other  kind  of  compound  thermoelectric  couple,  which  in 
the  following  form  appears  to  be  due  to  Bristol,  was  designed 
for  the  elimination  of  a  portion  of  the  expensive  platinum  and 
platinum-rhodium  wires.  It  consists  in  the  substitution  of  inex- 
pensive alloys  for  the  part  of  the  couple  which  is  not  exposed  to 


i76 


HIGH  TEMPERATURES 


a  temperature  above  a  red  heat,  as  shown  in  Fig.  50,  these  alloys 
being  so  chosen  as  to  give  the  same  E.M.F.-temperature  relation 
as  the  platinum-rhodium  couple;  so  that  the  resultant  E.M.F. 
generated  by  the  compound  couple  is  the  same  as  if  the  entire 
couple  were  of  platinum  and  platinum-rhodium. 


Compound  Thermo-Electric 
Couple v 


Inexpensive  Substitute 

for  Platinum-Rhodium 

Couple 


Leads  to  Indicating 
Instrument 

Fig.  50.     Bristol's  Compound  Couple. 

In  England,  Peake  proposes  to  use,  with  platinum-indium  or 
rhodium  couples,  such  compensating  leads,  made  with  one  wire 
of  copper  and  the  other  of  cupronickel  (Ni  from  o.i  to  5  per 
cent) .  That  good  compensation  may  be  obtained  by  this  method 
is  shown  by  the  following  table  from  products  used  by  the  Cam- 
bridge Scientific  Instrument  Company: 

PEAKE'S  COMPENSATING  LEADS. 


Temperature. 

Pt-Pt-Ir 
couple. 

Compensating 
leads. 

°C. 
o 

Millivolts. 
O 

Millivolts. 
O 

50 

0-59 

0.60 

IOO 

1-25 

1-25 

150 

200 

i-95 
2.68 

I.  QO 
2  .60 

250 

3-42 

3-40 

300 

4.20 

4-25 

The  compensating  couple  of  Chauvin  and  Arnoux  is  the  result 
of  an  attempt  to  eliminate  the  considerable  length  of  wire  and 
relatively  high  resistance  of  the  platinum  thermocouple,  and  at 


THERMOELECTRIC  PYROMETER 


177 


the  same  time  keep  the  advantages  of  such  a  couple  for  industrial 

measurements  to  1600°  C.  with  a  pivot  instrument  of  robust 

type  in  those  cases  where 

it  is  not  necessary  to  have 

a    considerable    length    of 

wire  exposed  to  the  hottest  pt   /  \  rt  ir 

temperatures.     The  ar- 

rangement  shown   in  Fig. 

51    consists   in   measuring 

the    temperature    in    two 

steps.      The   platinum- 

iridium  couple  is  in  series 

with  one  of  iron-constantan, 

whose  hot  junction  is  placed 

beside  the  cold  junction  of 

the  platinum  couple.     On 

account  of  the  greater 

E.M.F.    of    the    iron-con- 

stantan couple,  it  is  neces- 

sary to  shunt  this  last  with 

a   resistance   to    give    the 

same  temperature   difference.     This  shunt  reduces  the  total 

resistance  and  so  facilitates  the  use  of  long  canes. 

Referring  to  the  figure,  we  have  for  the  platinum-indium 
couple: 


Fig.  51- 


Furnace 


Porcelain 
Cane 


Iron 


Cane 


Handle 


Leads 


Galvanometer 
Compound  Couple  of  Chauvin 
and  Arnoux. 


and  for  the  iron-constantan  couple  : 

*=/'(*-*). 

Now,  if  the  alloys  are  properly  chosen  in  composition,  it  ap- 
pears to  be  possible  to  so  adjust  the  shunt  as  to  obtain  E.M.F.- 
temperature  curves  superposable  for  the  two  couples  between  0 
and  /,  which  is  equivalent  to  having  a  single  platinum-indium 
couple,  whence: 


178  HIGH  TEMPERATURES 

The  value  of  the  shunt  resistance  can  be  shown  to  depend  only 
on  the  resistance  of  the  compensating  couple  and  on  the  ratio  of 
the  two  thermoelectric  powers. 

The  temperature  6  should  not  exceed  800°  C.,  and  a  very  pure 
iron  tube  should  be  used  to  prevent  anomalies  around  700°  C. 

Calibration  of  Thermocouples.  —  With  the  several  national 
standardizing  laboratories  organized  and  equipped  for  such  work 
as  the  calibration  of  pyrometers,  it  is  no  longer  necessary  for  an 
individual  to  concern  himself  with  this  matter.  Nevertheless,  it 
is  often  desirable  to  be  able  to  carry  out  such  calibrations  simul- 
taneously with  other  investigations  or  with  the  apparatus  in 
hand.  We  shall  not  concern  ourselves  with  descriptions  of  the 
calibration  of  the  electrical  measuring  apparatus  used,  such  as 
millivoltmeters  and  potentiometers;  they  will  be  found  in  any 
book  on  the  testing  of  electrical  apparatus.  It  is  rare  that  an 
individual  is  so  situated  as  to  have  the  equipment  necessary  for 
these  electrical  calibrations,  when  recourse  must  be  had  to  the 
standardizing  laboratories.  In  the  case  of  thermocouples,  how- 
ever, used  with  pyrometer  galvanometers  possessing  a  millivolt 
scale  which  is  approximately  correct  or  a  scale  of  equal  parts,  the 
calibration  of  the  thermocouple  and  galvanometer  may  be  effected 
simultaneously  to  a  relatively  high  degree  of  accuracy  by  taking 
a  sufficient  number  of  fixed  points  with  the  couple  joined  to  its 
galvanometer. 

We  shall  assume,  therefore,  either  that  the  measuring  appa- 
ratus is  correct  or  that  it  may  be  sufficiently  well  calibrated  by 
the  operations  carried  out  on  the  thermocouple. 

We  shall  consider  first  the  requirements  for  the  highest  accuracy 
attainable  with  platinum-metal  thermocouples  suitable  for  high 
temperatures,  and  then  methods  applicable  to  industrial  practice 
and  for  base-metal  couples. 

Precision  Calibration.  —  We  have  seen  that  for  an  accuracy  of 
better  than  5  degrees,  or  in  many  cases  of  10  degrees,  it  is  neces- 
sary to  use  the  potentiometric  method  of  measurement.  The  cold 
junctions  of  the  couple  should  be  kept  at  o°  C.,  and  the  wires 
of  the  couple  should  be  annealed  and  shown  to  be  sufficiently 


THERMOELECTRIC   PYROMETER  179 

homogeneous.  Only  couples  of  the  platinum  metals  should  be 
used,  at  least  for  temperatures  above  600°  C. 

The  calibration  may  be  carried  out  by  the  use  of  melting  and 
boiling  points  of  known  values,  or  by  comparison  in  a  suitable 
furnace  with  one  or  more  standardized  pyrometers.  Great  care 
has  to  be  taken  with  the  insulation  of  the  thermocouple  circuit, 
especially  at  high  temperatures  and  when  electric  heating  is  used. 
Reversal  of  the  heating  circuit  will  show  this  effect.  It  is  also 
necessary  to  insure  sufficient  depth  of  immersion  in  the  bath  or 
furnace  to  avoid  errors  due  to  heat  conduction  along  the  wires 
of  the  couple.  This  can  be  tested  by  changing  the  depth  of  im- 
mersion in  the  region  of  constant  temperature,  when  the  readings 
should  not  change,  provided  of  course  the  wires  are  homogeneous. 

Crucible  Method.  —  In  the  case  of  the  metals  or  salts  which 
are  used  to  give  the  temperature  of  their  freezing  or  melting 
points,  it  is  usually  better  in  exact  work  to  use  crucibles  con- 
taining a  considerable  quantity  of  the  material,  300  c.c.  or  more, 
although  a  skilled  observer  using  a  suitable  furnace  can  get  good 
results  with  very  small  quantities  of  material.  Either  gas  or  elec- 
tric furnaces  may  be  used.  The  latter  permit  a  more  delicate 
control  both  of  the  rate  of  cooling  and  of  the  atmosphere  in 
which  the  melting  or  freezing  is  carried  out,  but  the  former  can 
usually  be  heated  much  more  rapidly.  For  work  to  i°  it  is  better 
to  keep  to  the  electric  furnace. 

As  to  the  choice  between  melting  and  freezing  points,  the 
consensus  of  opinion  is  in  favor  of  the  latter  when  possible  as 
being  usually  sharper,  although  sometimes  complicated  by  under- 
cooling, as  in  the  case  of  the  metals  antimony  and  tin.  With 
some  salts  this  phenomenon  is  prohibitive  of  using  the  freezing 
point. 

The  crucibles  should  of  course  be  of  material  that  does  not 
react  with  the  charge  or  the  atmosphere  of  the  furnace,  dissolve 
in  the  former,  or  let  the  furnace  atmosphere  penetrate  into  the 
charge  when  they  react  with  each  other.  For  salts,  the  best 
material  for  the  crucible  is  platinum,  but  nickel  crucibles  will 
also  answer  in  many  cases,  and  they  are  inexpensive.  Fire-clay 


l8o  HIGH  TEMPERATURES 

crucibles,  and  even  those  of  porcelain,  may  be  used  with  certain 
salts.  For  the  nonoxidizable  metals  there  are  several  substances 
available,  such  as  porcelain,  magnesia,  lime,  alumina,  graphite, 
and  quartz.  The  oxidizable  metals  which  do  not  dissolve  graph- 
ite are  best  melted  in  graphite  crucibles,  those  of  the  Acheson 
Company  being  almost  pure  graphite.  A  crucible  only  partly  of 
graphite,  such  as  the  Dixon  crucibles,  is  often  sufficient,  and  lasts 
longer  than  those  of  pure  graphite,  and  is  to  be  preferred  when  a 
gas  furnace  is  used.  These  crucibles  should  have  covers,  and  in 
addition  the  surface  of  the  metal  should  be  covered  with  powdered 
graphite.  In  some  cases  a  gas,  such  as  CO  or  H,  which  acts  as  a 
reducing  agent,  or  prevents  oxidation  such  as  N,  must  be  led  into 
the  furnace.  With  the  gas  furnace,  it  is  well  to  prevent  the  direct 
play  of  the  flames  on  the  crucible  by  surrounding  the  latter 
with  a  cylinder  of  metal  such  as  wrought  iron,  which  helps 
equalize  temperature  within  the  crucible. 

There  are  a  considerable  number  of  makers  of  crucible  furnaces, 
both  gas  and  electric,  suitable  for  ordinary  freezing-point  deter- 
minations. For  the  attainment  of  the  higher  temperatures 
with  the  former  an  air  blast  is  necessary,  and  for  the  latter  a 
rheostat  and  ammeter  or  voltmeter  are  necessary  auxiliaries. 

The  electric  method  of  heating  was  first  used  in  pyrometric 
work  for  the  determination  of  fixed  points  by  means  of  the  ther- 
mocouple by  D.  Berthelot  in  France  and  Holborn  and  Day  in 
Germany.  The  earlier  furnaces  were  constructed  by  winding 
pure  nickel  or  platinum  wire  on  porcelain  tubes  inclosed  in  an 
outer  tube  of  porcelain  and  wrapped  in  asbestos.  The  nickel- 
wound  furnaces  may  be  used  up  to  1300°  C.  for  a  short  time  with 
care  and  they  are  readily  rewound  when  burnt  out.  Their  life 
is  prolonged  by  packing  the  wire  so  as  to  prevent  access  of  air. 
The  platinum-wire  furnaces  are  very  expensive,  but  may  be  used 
up  to  1500°  C.  These  last  have  since  been  pretty  generally  dis- 
placed by  furnaces  of  the  Heraeus  type,  which  are  made  by  wind- 
ing platinum  foil  of  about  0.007  mm.  thickness  on  porcelain  tubes 
covered  with  an  aluminium  earth  paste  which  does  not  attack 
platinum  at  high  temperatures.  These  furnaces  are  inexpensive 


THERMOELECTRIC  PYROMETER 


181 


and  very  durable  to  1300°  when  carefully  used.  Heraeus  also 
manufactures  iridium  resistance  furnaces  with  which  tempera- 
tures over  2000°  C.  may  be  reached,  and  a  very  constant  tempera- 
ture maintained.  With  this  type,  special  precautions  have  to 
be  taken  to  prevent  the  evaporation  of 
iridium  on  to  thermocouple  wires.  A 
further  advantage,  in  some  cases,  of  the 
electric  furnace  is  the  absence  of  reduc- 
ing gases. 

The  use  of  electric  heating  has  rendered 
the  standardization  of  the  thermocouple 
and  all  other  pyrometers  a  relatively 
easy  matter  and  increased  greatly  the 


Fig.  52.  Electrical  Crucible  Furnace. 


Fig.  53.  Electrical  Crucible  Furnace. 


accuracy  and  range  attainable  in  establishing  the  fixed  points 
in  pyrometry. 

Types  of  electric  crucible  furnaces  are  shown  in  Figs.  52,  53, 
and  175.  In  those  of  the  Geophysical  Laboratory  (52  and  53), 
the  platinum  heating  coil  is  embedded  in  Marquardt  mixture. 

Among  the  gas  crucible  furnaces  we  may  mention  those  of  the 


182 


HIGH   TEMPERATURES 


Buffalo  and  the  White  Dental  companies,  the  American  Gas 
Furnace  Company,  and  the  Meker  furnaces.  A  form  of  furnace 
that  Le  Chatelier  found  very  serviceable  is  shown  in  Fig.  54. 
It  is  a  furnace  of  English  design,  which  has  the  advantage  to 

resist  almost  indefinitely  the  ac- 
tion of  heat  and  to  be  very  easily 
repaired.  The  principle  of  the 
construction  of  these  furnaces  is 
to  make  them  of  two  concentric 
layers.  The  outer  covering  of 
fire  clay,  bound  together  by  iron, 
gives  solidity  to  the  furnace;  it 
receives  but  indirectly  the  action 
of  the  heat,  and  is  not  exposed 


Fig.  54.     Gas  Crucible  Furnace. 


to  cracking  by  shrinkage  under  the  action  of  too  high  tempera- 
tures. The  inner  envelope,  which  alone  receives  the  action  of 
the  heat,  is  made  of  large-grained  quartz  sand,  grains  of  i  mm., 
mixed  with  a  small  amount  of  a  flux.  At  a  high  temperature  the 
quartz  does  not  shrink  as  does  clay;  it  expands,  on  the  contrary, 
passing  over  to  the  form  of  amorphous  silica  with  a  change  of 
density  from  2.6  to  2.2.  But  this  transformation  is  effected  only 
with  extreme  slowness,  otherwise  it  would  burst  the  furnace. 
If  by  chance  this  inner  lining  falls  down,  it  is  easily  replaced 
by  putting  into  the  furnace  a  glass  jar  of  suitable  diameter,  sur- 
rounded with  a  sheet  of  oiled  paper,  and  packing  about  this 
coarse  quartz  sand  slightly  moistened  with  a  sirupy  solution  of 
alkaline  silicate.  The  furnace  is  heated  by  means  of  a  lateral 
opening  with  a  Fletcher  lamp,  which  has  the  advantage  of  being 
sturdy,  or  with  an  ordinary  blast  lamp. 

Among  the  metals  which  can  be  used,  or  which  are  not  too 
expensive  for  the  calibration  of  thermocouples  by  the  crucible 
method,  are  Sn,  Cd,  Pb,  Zn,  Al,  Sb,  Ag,  Cu,  Ni,  Fe,  and  Co.  The 
last  four  are  readily  oxidizable,  as  is  also  Sb,  but  its  oxide  does 
not  appear  to  dissolve  in  the  metal;  while  Ag  absorbs  oxygen 
from  the  air  unless  protected.  Al  attacks  crucibles  containing 
silica,  and  is  difficult  of  manipulation.  The  behavior  of  these 


THERMOELECTRIC   PYROMETER 


metals  is  discussed  at  length  in  Chapter  XI.  If  only  three 
points  are  required,  Zn,  Sb,  and  Cu  will  suffice  if  the  last  two 
are  manipulated  in  a  strictly  reducing  atmosphere.  It  is,  of 
course,  absolutely  essential  that  the  purity  of  the  metals  used 
can  be  vouched  for.  The  eutectic  Cu-Cu2O  is  well  fixed,  as  is  also 
the  alloy  Cu2-Ag3,  to  serve  for  thermo-couple  calibrations. 

The  exact  values  of  the  freezing  point  of jvery  few  salts  are 
well  known,  as  is  shown  on  page  190.  NaCl  is  perhaps  the 
most  certainly  determined  and  is  conveniently  located 
between  Sb  and  Cu  to  serve  as  a  fourth  calibration 
point;  other  well-known  salts  are  Na2SO4  and  chem- 
ically prepared  diopside  (CaSiO3  »MgSi03).  A  thin 
platinum  detachable  crucible  used  at  the  Geophysical 
Laboratory  with  small  quantities  of  salt,  into  which 
the  couple  may  be  dipped,  is  shown  in  Fig.  55. 

Some  of  the  salts  will  attack  the  porcelain  sheath 
about  the  thermocouple.  A  metal  one,  platinum  or 
nickel,  may  be  substituted,  taking  care  to  keep  the 
wires  of  the  thermocouple  insulated.  The  metals  act 
variously  on  the  porcelain  tubes.  If  left  in  zinc  they 
will  invariably  break  on  cooling.  If  withdrawn  from 
liquid  aluminium,  unless  greatly  overheated,  they  will 
likewise  break  if  any  metal  adheres  to  the  porcelain. 
The  Al  also  dissolves  silica.  They  may  be  left  in 
copper,  however,  and  reheated  without  cracking. 
The  best  practice  is  to  always  withdraw  from  the 
liquid  the  porcelain  or  other  protecting  tube  with- 
out any  substance  sticking  to  it  after  finishing  the 
observations  with  any  metal  or  salt.  The  use  of  Detachable 
quartz-glass  protecting  tubes  will  in  general  prove  Platinum 
disappointing.  Crucible. 

Wire  Method.  —  Measurements  of  the  melting  points  of  a  pre- 
cision of  one  or  two  degrees  may  be  obtained  with  the  nonoxidiz- 
able  metals,  which  can  be  drawn,  such  as  Au  (1063°),  Pd  (1550°), 
and,  to  a  slightly  less  precision,  Pt  (1755°),  by  inserting  a  short 
length  of  wire  between  the  two  wires  of  the  hot  junction  of  the 


184 


HIGH  TEMPERATURES 


thermocouple  and  gradually  raising  the  temperature  until  the  cir- 
cuit breaks,  due  to  the  melting  of  the  interposed  link,  and  noting 
the  maximum  reading.  Only  a  millimeter  or  two  of  the  couple 
wire  need  be  lost  by  this  operation.  The  best  results  are  obtained 
in  a  clear  resistance-tube  furnace  of  small  diameter;  the  Pt  point 


Water— * 


CO— *^   V^T  Thermoelement 
Fig.  56.     Sosman's  Carbon  Furnace. 

cannot  be  obtained  in  this  way  except  in  some  special  form  of 
carbon  furnace  (Fig.  56),  or  in  an  iridium-  or  platinum-alloy 
furnace  (see  Fig.  174,  Chap.  XI).  The  wires  should  be  pro- 
tected by  porcelain,  quartz-glass,  or  magnesia  tubes.  In  any  case 
the  link  should  be  fused,  not  tied  into  place,  and  is  best  of  nearly 
the  same  diameter  as  the  couple  wires.  Very  great  precautions 


THERMOELECTRIC  PYROMETER 


185 


have  to  be  taken  to  guard  against  leaks  from  the  furnace,  it  being 
absolutely  necessary  that  no  part  of  the  thermocouple  circuit 
touch  any  hot  part  of  the  furnace  in  the 
case  of  Pd  and  Pt  melts,  and  the  insulators 
of  the  separate  leads  should  not  touch 
each  other  within  the  furnace. 

In  general,  much  less  reliable  results  will 
be  obtained  when  the  oxyhydrogen  or 
other  flame  has  to  be  used.  The  unstead- 
iness of  the  flame  may  be  partly  overcome 
by  immersing  the  linked  or  wrapped  junc- 
tion in  a  small  muffle  or  in  a  small  crucible 
containing  powdered  refractory  material 
such  as  alumina.  For  the  platinum  point 
the  oxyhydrogen  flame  is  required;  the 
palladium  point  can  be  obtained  with  a 
strong  blast,  and  the  gold  point  with  an 
ordinary  Bunsen.  In  industrial  plants 
advantage  may  be  taken  of  flues,  furnaces, 
etc.,  to  give  the  requisite  heat. 

The  precision  obtainable  with  the  wire 
method  is  illustrated  by  observations  of 
Waidner  and  Burgess  on  the  melting 
point  of  palladium,  carried  out  in  an  elec- 
tric resistance  furnace  (Fig.  57),  the  palladium  wire  being  inserted 
at  the  junctions  of  a  series  of  Pt-Rh  and  Pt-Ir  couples.  The 
temperature  scale  of  the  Table,  page  186,  is  that  of  the  thermo- 
couple (equation  (3),  page  112). 

Instead  of  inserting  the  link  in  the  thermocouple  circuit,  it 
may  be  put  in  a  neighboring  auxiliary  circuit  containing  some 
electrical  device  for  recognizing  a  break.  The  accuracy  will  be 
somewhat  lessened,  but  the  protected  couple  will  remain  intact 
by  this  modification. 

A  wire  method  devised  by  Wright  suitable  for  use  with  minute 
quantities  of  salts  is  shown  in  Fig.  58.  The  salt  is  mounted  at  T 
on  the  slightly  flattened  junction  within  a  water-jacketed  electric 


Fig.  57.     Mounting  for 
Wire  Method. 


1 86 


HIGH  TEMPERATURES 


furnace  placed  below  a  microscope,  by  means  of  which  the  melt- 
ing or  freezing  is  observed. 


Fig.  58.     Furnace  and  Microscope  for  Minute  Pieces. 
MELTING  POINT  OF  PALLADIUM  —  WIRE  METHOD. 


Thermo- 
couple. 

Number  of 
observations. 

Melting  point 
of  palladium. 

Remarks. 

Wi 

sz 

W3 
P* 
Pl 

S2 

P>2 

w> 

w, 

6 
4 

5 

2 
2 

2 

3 
6 

1531.0° 
1530.5 
1530.0 

I529-5 
1530.0 

1530.5 
1530.0 

1530.5 
I530.I 

Horizontal  furnace;  bare  wires. 
Horizontal  furnace;  porcelain  tubes. 

«                          <  i 
Vertical  furnace;  see  Fig.  57. 

M.P.  on  thermoelectric  scale  (equa- 
tion (3),  p.  112). 

Mean  =  1530.  2° 

THERMOELECTRIC  PYROMETER 


I87 


Boiling  points,  including  those  of  the  metals  such  as  Cd  and 
Zn,  have  frequently  been  used  for  the  calibration  of  thermo- 
couples, but  the  boiling  metals  are  much  more  difficult  to  manip- 
ulate than  the  melting,  and  there  is  far  greater  danger  of  con- 
tamination of  the  thermocouples,  nor  is  there  need  of  resorting 
to  them.  If  desired,  however,  the  freezing  points  of  Sn,  Pb  or 
Cd,  and  Zn  may  be  replaced  by  the  boiling  points 'of  naphthaline, 
benzophenone,  and  sulphur  respectively,  none  of  which  attack 
the  couples  ordinarily  used.  The  standard  form  of  boiling  appa- 
ratus for  an  accuracy  of  0.05°  C.  or  better  is  shown  in  Fig.  169, 
except  that  for  naphthalene  and  benzophenone  a  side  condenser 
tube  should  be  added ;  or  an  air  blast  from  a  ring  burner  around 
the  top  of  the  boiling-tube  may  be  used. 

For  a  somewhat  less  accuracy  the  smaller  portable  apparatus 
of  Barus  (Fig.  59)  may  be  used  for  boiling  points,  including  also 
water  and  analine.  This  consists  of  a  tube  of 
thin  glass,  similar  to  test  tubes,  of  15  mm. 
inside  diameter,  300  mm.  long,  with  a  small 
bulb  at  50  mm.  below  the  open  end.  It  is 
surrounded  with  a  plaster  muff  of  150  mm. 
height  and  100  mm.  diameter  which  has  been 
cast  about  the  glass  tube  inside  of  a  thin 
metallic  cylinder  forming  the  outside  surface. 
The  bulb  is  immediately  above  the  plaster 
jacket,  below  which  the  tube,  closed  at  its 
lower  end,  extends  to  a  distance  of  70  mm. 
As  soon  as  the  plaster  has  begun  to  set  the 
glass  tube  is  taken 'out,  giving  it  a  slight 
twisting  motion.  The  cylinder  is  left  to  dry, 
and  the  tube  is  again  put  in  place.  This  allows,  Fjfar£us 
when  the  tube  is  broken,  of  taking  it  out 
and  replacing  it,  which  would  be  difficult  if  it  adhered  to  the 
plaster.  A  jacketed  Victor  Meyer  tube  may  also  be  used. 

The  lower  free  portion  is  heated  by  a  Bunsen  flame  gently  at 
first,  then  without  any  special  precaution,  once  boiling  sets  in. 
The  liquid  at  rest  should  occupy  two-thirds  of  the  height  of  the 


i88 


HIGH  TEMPERATURES 


free  end  of  the  tube.  The  heating  is  continued  until  the  liquid 
coming  from  the  condensation  of  the  vapor  runs  abundantly 
down  the  walls  of  the  bulb.  The  flame  is  then  adjusted  so  that 
the  limit  of  condensation  of  the  liquid,  which  is  very  sharp, 
remains  constantly  midway  up  the  bulb.  There  is  then  a  very 
uniform  temperature  in  the  interior  of  the  glass  tube  throughout 
the  height  of  the  plaster  cylinder.  The  junction  of  the  couple 
is  inserted  and  the  coil  of  the  galvanometer  takes  up  a  fixed 
invariable  position.  It  is  well  to  prevent  the  liquid  from  run- 
ning down  about  the  couple  by  placing  a  small  cone  of  aluminium 
or  asbestos  above  the  junction.  Electric  heating  may  also  be 
used. 

For  the  boiling  point  of  zinc,  Barus  made  small  crucibles  of 
porcelain  very  ingeniously  arranged,  but  also  very  complicated, 
besides  being  fragile  and  costly.  One  can 
make  use  more  simply  of  a  porcelain 
crucible  70  mm.  deep  (Fig.  60),  filled 
with  melted  zinc  for  50  mm.  of  its  depth, 
and,  above,  20  mm.  of  charcoal  dust. 
A  cone  pierced  with  a  central  hole  lets 
pass  a  small  porcelain  tube  containing 
the  couple.  The  whole  is  heated  until 
there  is  seen  a  small  white  flame  of  zinc 
escaping  from  the  crucible.  It  Is  indis- 
pensable that  the  openings  for  the  escape 
of  zinc  vapor  be  large  enough.  They 
tend)  indeed,  to  become  clogged  by  a 
deposit  of  zinc  oxide  which  solders  at 
the  same  time  the  cover  to  the  crucible, 
and  this  causes  an  explosion  when  there  is  no  longer  vent  for 
the  zinc  vapors.  A  better  form  providing  for  the  condensation 
of  vapors  was  used  by  D.  Berthelot. 

Technical  Calibrations.  —  The  process  of  calibration  is  greatly 
simplified  if  an  uncertainty  of  5°  C.  or  more  may  be  permitted, 
as  is  the  case  for  most  technical  operations.  When  the  crucible 
method  is  used,  smaller  crucibles  and  furnaces  may  be  allowed 


Fig.  60.    Zinc-boiling 
Apparatus. 


THERMOELECTRIC  PYROMETER  189 

than  for  exact  calibration,  and  metals  and  salts  of  less  certain 
purity  may  be  tolerated,  although  it  is  most  certainly  safer  to 
use  only  very  pure  materials.  Some  metals,  such  as  Al  and  Sb, 
have  their  melting  points  greatly  influenced  by  small  impurities, 
and  are  obtainable  in  sufficient  purity  with  difficulty.  Other 
metals,  such  as  Sn,  Pb,  Zn,  and  Cu,  can  be  trusted  from  almost 
any  source  of  supply  to  give  temperatures  to  within  2°  C.  of  the 
melting  points  of  the  pure  metals.  The  precautions  of  manipu- 
lation mentioned  in  the  preceding  paragraphs  apply  here,  with 
somewhat  attenuated  emphasis,  depending  upon  the  accuracy 
desired.  For  example,  it  is  often  not  convenient  to  maintain 
the  cold  junctions  at  o°  C.,  or  one  may  desire  to  keep  them  during 
calibration  at  the  average  temperature  to  which  they  are  sub- 
jected in  use  or  attached  directly  to  the  pyrometer  galvanometer. 
We  have  indicated  elsewhere  (page  155)  how  to  make  allowance 
for  variations  in  cold-junction  temperatures. 

The  galvanometer  and  thermocouple  may  be  tested  either 
together  or  separately,  but  the  former  method  is  the  more  con- 
venient and  eliminates  the  use  of  any  auxiliary  apparatus.  Five 
or  six  points  on  the  galvanometer  scale  are  usually  sufficient  for 
a  technical  calibration  of  the  couple  and  galvanometer  together. 
These  may  be  given  by  any  of  the  freezing  or  boiling  points 
we  have  mentioned.  A  convenient  inexpensive  series  of  the 
former,  requiring  the  minimum  of  precautions  in  manipulation 
and  suitable  in  crucibles  of  50  or  100  c.c.  in  a  small  air-gas 
furnace  when  the  couples  are  wires  of  small  diameter,  is  the 
following : 

Sn 232°  C.  Al 657°  C. 

Pb 327  NaCl 800 

Zn 419  Cu  •  CuO2 1063 

The  last  is  copper  saturated  with  its  oxide.  A  piece  of  carbon 
steel  (0.9  per  cent  C)  bored  with  a  small  hole  into  which  the 
couple  is  inserted  will  give  on  cooling  a  calibration  temperature 
of  about  700°  C. 

If  it  is  desired  to  use  salts  only,  as  may  be  the  case  with  cer- 
tain base-metal  couples  of  the  cane  type,  which  are  plunged  with- 


I QO  HIGH  TEMPERATURES 

out  protection  directly  into  the  bath,  there  will  be  some  sacrifice 
of  accuracy  even  when  large  quantities  of  salt  are  used  in  nickel 
crucibles,  due  in  part  to  our  uncertainty  of  their  melting  points, 
in  part  to  the  great  effects  that  slight  impurities  exercise  on  these 
temperatures,  and  above  all  to  heat  conduction  along  the  leads, 
as  the  couple  is  practically  short-circuited  by  the  salt  near  its 
surface.  This  is  even  more  emphatically  true  when  a  couple  is 
plunged  bare  into  metal,  which  is  bad  practice  even  when  the 
couple  will  stand  it.  The  following  list  of  salts  is  suggested,  with 
considerable  reserve  as  to  the  numerical  values,  some  of  which 
may  be  10°  C.  or  more  in  error: 

NaNO3 308°  C.        BaCl2 950°  C. 

KN03 336  K2S04 1060 

Ca(NO3)2 550  Na2SiO3 1088 

KI 680  Li2SiO3 1202 

KC1 780  Diopside 1391 

J   NaCl 800  Anor^hite J55O 

^'  Na2SO4 885 

Another  sharply  defined  temperature  which  may  be  used  is 
the  transformation  temperature  of  crystalline  quartz  at  575°  C., 
obtained  with  a  good  quality  of  silica  sand. 

Still  another  method  of  calibration  is  by  the  comparison  of  the 
pyrometer  readings  with  those  of  a  standardized  instrument  in 
the  same  furnace.  With  the  platinum  couples  and  a  porcelain- 
tube  platinum-resistance  furnace  of  1.5  cm.  diameter  and  30  to 
60  cm.  long,  results  to  2  degrees  or  better  may  be  obtained  if 
special  precautions  are  taken  to  insure  constant  temperature, 
such  as  inclosing  hot  junctions  within  a  short  platinum  cylinder 
and  passing  them  through  notches  in  a  platinum  disk.  When 
base-metal  couples  are  compared  with  those  of  platinum,  it  is 
usually  necessary  to  protect  the  latter  from  contamination  from 
the  former  by  inclosing  the  platinum  wires  and  junctions  in 
glazed  porcelain  or  other  suitable  tubes.  A  better  temperature 
distribution  for  the  comparison  of  base-metal  couples  than  is 
found  usually  in  tube  furnaces  may  be  obtained  by  using  large 
crucible  furnaces  in  which  are  placed  baths  of  mixed  salts  which 
may  be  stirred  and  contained  in  long  iron  crucibles. 


THERMOELECTRIC  PYROMETER  191 

Industrial  and  Scientific  Applications.  —  The  measurement  of 
temperatures  by  thermoelectric  couples  has  enhanced  the  accu- 
rate knowledge  of  a  great  number  of  high  temperatures  of  which 
previously  little  or  nothing  was  known.  The  earlier  measure- 
ments were  particularly  numerous  in  the  scientific  and  industrial 
investigations  on  iron.  It  was  with  the  thermoelectric  couple 
that  Osmond  and  others,  Roberts-Austen,  Arnold,  Howe,  and 
Charpy  made  all  their  studies  on  the  molecular  transformations 
of  irons  and  steels.  The  conditions  of  manufacture  and  of 
treatment  of  these  metals  have  been  improved  by  the  introduc- 
tion into  industrial  works  of  this  method  of  high-temperature 
measurements. 

We  give  below,  as  examples  of  the  early  use  of  the  thermo- 
couple, a  series  of  determinations  made  by  Le  Chatelier  in  a 
number  of  industrial  operations. 

Steel.  —  Siemens-Martin  open-hearth  furnace : 

Gas  at  the  outlet  of  the  gas  generator 720° 

Gas  at  the  entrance  of  the  regenerator 400 

Gas  at  the  outlet  of  the  regenerator 1200 

Air  at  the  outlet  of  the  regenerator 1000 

Interior  of  the  furnace  during  refining 1550 

Smoke  at  the  foot  of  the  chimney 300 

Glass. — Basin  furnace  for  bottles;  pot  furnace  for  window 
glass: 

Furnace 1400° 

Glass  in  affinage 1310 

Annealing  of  bottles 585 

Rolling  of  window  glass 600 

'  Illuminating  gas.  —  Gazogene  furnace: 

Top  of  furnace 1 190° 

Base  of  furnace 1060 

Retort  at  end  of  distillation 975 

Smoke  at  base  of  regenerator 680 

Porcelain.  —  Furnaces: 

Hard  porcelain 1400° 

China  porcelain 1275 

To-day,  the  use  of  the  thermocouple  in  the  most  varied  indus- 
tries is  so  widespread  that  the  above  list  could  be  indefinitely 


IQ2  HIGH  TEMPERATURES 

multiplied.  It  is  not  merely,  however,  in  the  determination 
of  the  temperatures  entering  into  a  great  number  of  industrial 
operations  that  the  thermoelectric  pyrometer  has  paved  the  way 
for  itself  and  for  other  types  of  pyrometer,  but  it  is,  above  all,  in 
the  ability  that  is  thereby  given  by  such  temperature  measure- 
ments to  control  the  quality  of  the  products  of  these  operations 
depending  on  temperature,  and  so  permit  the  exact  reproduction, 
within  as  close  limits  as  is  wanted,  of  any  desired  result,  and 
to  increase  thereby  enormously  the  efficiency  of  many  industrial 
plants. 

The  use  of  the  thermocouple  in  scientific  investigations  has  been 
not  less  extensive  or  fruitful,  and  we  have,  for  instance,  what  may 
be  called  a  new  science,  or  at  least  a  new  aspect  of  chemistry, 
namely,  thermal  analysis,  which  has  grown  up  in  recent  years, 
based  mainly  on  the  interpretation  of  physicochemical  phenom- 
ena at  high  temperatures  by  means  of  the  indications  of  the  ther- 
mocouple. 

In  the  development  of  scientific  metallurgy,  again,  the  thermo- 
couple has  been  almost  the  only  temperature-measuring  device 
which  has  been  employed.  These  two  generalizations  are  suffi- 
cient to  indicate  its  secure  position  as  an  instrument  of  pyrometric 
research. 

Conditions  of  Use.  —  Thermoelectric  couples,  as  we  have  seen, 
may  be  divided  for  convenience  into  two  general  classes,  the 
platinum-alloy  couples  and  the  base-metal  couples,  both  of  which 
are  readily  calibrated  and  easy  to  use.  The  former,  by  reason  also 
of  their  small  size  and  of  the  permanence  and  precision  of  their 
indications,  are  on  the  whole  preferable  to  all  other  pyrometric 
methods  for  ordinary  investigations,  scientific  or  industrial,  over 
the  wide  temperature  range  for  which  they  are  best  adapted, 
or  from  300°  to  1600°  C.  We  shall  see,  however,  that  when  the 
highest  accuracy  is  desired,  or  better  than  i°  C.,  the  resistance 
thermometer  of  platinum  may  be  given  the  preference  to  900°  C. 
from  the  very  lowest  temperatures,  even  over  the  thermocouple 
used  with  a  potentiometer.  The  former  also  is  somewhat  more 
adapted  for  use  with  robust  recording  instruments.  Above 


THERMOELECTRIC  PYROMETER  193 

1000°  C.,  however,  the  platinum  thermocouple  is  the  only  form 
of  electric  pyrometer  which  can  be  used  with  any  considerable 
certainty;  and  attached  either  to  a  suitable  direct-reading  galva- 
nometer or  to  an  automatic  recorder,  this  instrument  is  proving 
of  great  utility  in  the  industries.  It  may  also  be  wired  readily 
for  use  in  multiple  on  a  single  distant  recording  instrument,  each 
couple  also  having  its  separate  indicator  beside  it. 

For  temperatures  below  600°  C.,  there  is  gain  in  sensibility 
without  serious  loss  in  accuracy  by  substituting  such  couples  as 
that  of  silver-constantan  or  copper-constantan,  both  of  which  can 
be  kept  small;  and  below  500°  C.  we  reach  the  range  of  accurate 
mercury-in-glass  thermometers.  We  have  discussed  elsewhere 
the  precautions  and  methods  of  use  both  in  work  of  precision  and 
in  technical  work  for  the  various  types  of  couple. 

The  use  of  the  base-metal  couple  is  limited  to  the  technical 
field,  and  even  here  great  discernment  has  to  be  used  to  satisfy 
oneself  that  the  couple  in  hand,  with  its  accessories,  is  suitable 
for  the  use  to  which  it  is  contemplated  to  put  it.  There  are  few 
base-metal  couples  which  can  be  used  safely  above  1000°  C.,  and 
some  of  them  are  of  very  questionable  utility  for  any  purpose. 

We  shall  see  that,  even  in  the  range  for  which  it  is  best  adapted, 
the  thermocouple  may  in  certain  lines  of  work  be  replaced  to 
advantage  by  still  other  methods,  such  for  instance  as  the  radia- 
tion and  optical  pyrometers. 


CHAPTER  V. 
ELECTRICAL  RESISTANCE  PYROMETER. 

Introduction.  —  In  this  method  use  is  ordinarily  made  of  the 
variations  of  electric  resistance  of  a  platinum  wire  as  a  function 
of  the  temperature;  these  variations  are  of  the  order  of  mag- 
nitude of  those  of  the  expansion  of  gases.  The  ratio  of  the 
resistances  is  1.39  at  100°,  and  4.4  at  1000°.  As  electrical 
resistances  are  measurable  with  great  accuracy,  this  process 
of  estimation  of  temperatures  offers  a  very  great  sensibility, 
and  applying  exactly  the  law  that  connects  the  variation  of 
resistances  to  that  of  temperature  most  excellent  results  may 
be  obtained. 

The  electric  pyrometer  was  proposed  by  Siemens  in  1871 
(Bakerian  Lecture);  it  rapidly  came  into  use  in  metallurgical 
works  on  account  of  the  reputation  of  its  inventor,  but  it  was 
soon  abandoned  for  reasons  which  will  be  given  later.  This 
method  of  temperature  measurement  was  revived  twenty  years 
afterwards  by  Callendar  and  Griffiths,  and  has  been  growing  in 
favor  ever  since,  both  in  the  laboratory  and  in  the  industries, 
especially  in  England,  and  more  recently  in  America.  It  is 
perhaps  of  interest  to  note  that  in  Cambridge,  England,  the  re- 
sistance thermometer  was  first  brought  into  a  satisfactory  condi- 
tion as  a  physical  instrument  and  its  theory  successfully  worked 
out  by  Callendar  and  Griffiths;  there  it  was  first  used  in  most 
delicate  measurements  of  chemical  phenomena  by  Heycock  and 
Neville;  and  finally,  the  Cambridge  Scientific  Instrument  Com- 
pany were  pioneers  in  the  manufacture  of  instruments  suitable 
for  industrial  and  scientific  use. 

Work  of  Early  Investigators. —  Siemens. —  The  Siemens  pyrom- 
eter consists  of  a  fine  platinum  wire  i  m.  long  and  o.i  mm. 
in  diameter,  wound  on  a  cylinder  of  porcelain  or  fire  clay;  the 

194 


ELECTRICAL   RESISTANCE  PYROMETER  195 

whole  is  inclosed  in  an  iron  tube,  destined  to  protect  the  instru- 
ment from  the  action  of  the  flames. 

Siemens  tried  also,  but  without  success,  ceramic  materials 
impregnated  with  metals  of  the  platinum  group. 

To  measure  the  resistance  he  employed  either  a  galvanometer, 
for  laboratory  experiments,  or  a  voltameter,  for  the  measure- 
ments in  works.  In  this  latter  case  the  current  from  a  cell 
divides  between  the  heated  resistance  and  a  standard  resistance 
at  constant  temperature;  in  each  one  of  the  circuits  was  placed 
a  voltameter:  the  ratio  of  the  volumes  of  gas  set  free  gives  the 
ratio  of  the  current  strengths  and  thus  the  inverse  ratio  of  the 
resistances. 

Finally  Siemens  gave  a  formula  of  three  terms  connecting 
the  electrical  resistance  of  platinum  to  temperatures 'on  the  air 
thermometer,  but  without  publishing  the  experimental  data  on 
which  this  graduation  was  based. 

Experiment  soon  showed  that  the  apparatus  did  not  rest  com- 
parable with  itself.  A  committee  of  the  British  Association 
for  the  Advancement  of  Science  found  that  the  resistance  of 
platinum  increases  after  heating.  It  would  be  necessary  then 
to  calibrate  the  apparatus  each  time  that  it  was  used.  This 
change  of  resistance  is  due  mainly  to  a  chemical  alteration  of 
platinum,  which  is  enormous  when  heated  directly  in  the  flame, 
less,  but  still  marked,  if  placed  in  an  iron  tube,  and  which  almost 
disappears  if  use  is  made  of  a  platinum  or  porcelain  tube.  This 
increase  of  resistance  may  reach  15  per  cent  by  repeated  heatings 
to  900°. 

Platinum  being  very  costly  and  porcelain  very  fragile,  it  was 
impossible  to  use  these  two  bodies  in  the  industries,  which  alone 
at  that  time  occupied  themselves  with  measurements  of  high 
temperatures,  and  this  method  was  abandoned  completely  during 
twenty  years. 

Callendar  and  Griffiths.  —  These  savants  revived  this  method 
for  laboratory  purposes;  it  seems  the  best  for  many  kinds  of  work 
of  precision  to  moderately  high  temperatures,  on  the  condition  of 
being  assured  of  the  invariability  of  the  resistance  of  platinum. 


196  HIGH  TEMPERATURES 

Callendar  found  that  clay  helps  to  cause  the  variation  of 
resistance,  that  the  platinum  wire  becomes  brittle  on  its  support 
and  sticks  there;  this  action  is  probably  due  to  impurities  in  the 
clay.  With  mica,  on  the  other  hand,  which  the  wire  touches 
only  at  the  edges  (the  reel  is  made  of  two  perpendicular  slices  of 
mica) ,  there  is  perfect  insulation  without  cause  of  alteration ;  but 
mica  becomes  dehydrated  at  800°  and  then  becomes  very  fragile. 

All  metallic  solderings  should  be  proscribed,  for  they  are  "vola- 
tile and  attack  platinum. 

Pressure  joints  (screw  or  torsion)  are  equally  bad,  for  they 
become  loose.  One  should  use  only  autogenous  soldering  by  the 
fusion  of  platinum. 

Copper  conductors  should  also  be  rejected,  at  least  in  the 
heated  portions,  on  account  of  the  volatility  of  the  metal.  A 
pyrometer  with  such  conductors,  heated  during  an  hour  at  850°, 
showed  an  increase  of  resistance  of  J  per  cent. 

Holborn  and  Wien.  —  These  investigators  made  a  very  com- 
plete study  of  this  alterability  of  platinum  wires,  in  a  comparison 
between  the  methods  of  measurement  of  temperatures  by  electric 
resistance  and  thermoelectric  forces;  they  worked  with  wires  of 
o.i  mm.  to  0.3  mm.  diameter.  They  soon  found  that  above  1200° 
platinum  commences  to  undergo  a  feeble  volatilization  which 
suffices  to  increase  notably  the  resistance  of  the  very  fine  wires. 
Hydrogen  in  presence  of  silicious  materials  causes  at  about  850° 
a  rapid  alteration  of  the  platinum. 

Below  are  the  results  relative  to  wires  of  0.3  mm.  of  a  length 
of  1 60  mm.: 


Wire  a. 

R  at  15°. 

Wire  0. 

R  at  15". 

At  start  

o 

.  239  ohm 

At  start 

o 

24.7 

ohm 

After  heating  red-hot: 

After  several  days 

in  hy- 

.  -^tf./ 

Twice  in  air  at  1200°.  . 

0. 

238  ohm 

drogen  at  15°.  .  . 

o 

.246 

ohm 

Once  in  vacuo  at  1200° 

o 

,  240  ohm 

After  heating   in 

hydro- 

Once  in  H  at  1200°  .... 

o 

.  262  ohm 

gen  to  1200°.  .  .  . 

0 

•255 

ohm 

Once  in  vacuo  at  1200° 

0.253  ohm 

Wire  7. 

Rat  15°. 

At  start  .  . 

0.183 

ohm 

After  heating  in  air  to  1250°  (three  times) o.  182  ohm 

After  heating  in  H  to  1250° o.  188  ohm 

After  heating  in  H  to  1250° o.  195  ohm 


ELECTRICAL   RESISTANCE   PYROMETER  197 

Wire  7  heated  to  1350°  in  an  earthenware  tube  and  in  hydrogen 
became  brittle;  this  result  may  be  explained  by  a  siliciuration  of 
the  platinum,  for  there  is  nothing  observed  if  the  wire  is  heated 
by  the  electric  current  in  the  interior  of  a  cold  glass  tube,  even  in 
hydrogen.  Similar  experiments  were  made  by  the  same  observers 
with  palladium,  rhodium,  and  iridium.  We  shall  return  to  this 
question  of  the  constancy  of  the  resistance  of  platinum. 

Law  of  the  Variation  of  Platinum  Resistance.  —  Callendar 
and  Griffiths  have  compared  the  resistance  of  platinum  with  the 
air  thermometer  up  to  550°  C.;  they  found  that  up  to  500°  the 
relation  could  be  represented  at  least  to  0.1°  by  a  parabolic  for- 
mula of  three  parameters.  In  order  to  graduate  such  a  pyrometer 
it  would  be  sufficient  then  to  have  three  fixed  points:  ice,  steam, 
and  boiling  sulphur. 

They  gave  a  special  form  to  the  relation;  let  pt  be  the  platinum 
temperature  defined  by  the  relation 


. 

-K-100  ~~  -K-0 

that  is  to  say,  the  value  of  the  temperature  in  the  case  in  which 
the  resistance  varies  proportionally  to  the  temperature. 
They  then  placed 


It  would  appear  as  if  this  formula  contained  the  single  param- 
eter 5;  but  in  reality  pt  includes  two. 
Substituting  for  pt  its  value,  we  have 


.          *       WO  —        Q     .2 


an  equation  of  the  form 

Rt  =  RQ  (i  +  at  -  bf-), 

which  it  is  sometimes  convenient  to  use.  Callendar  and  Griffiths 
used  their  pyrometer  before  having  standardized  it  against  the 
air  thermometer.  Not  being  able  to  compute  t,  they  provisionally 
computed  the  approximate  temperatures  pt,  and  later  determined 


198 


HIGH  TEMPERATURES 


the  correction  between  /  and  pt}  after  having  sought  the  formula 
expressing  the  difference  between  these  two  quantities  by  means 
of  a  careful  determination  of  the  sulphur  boiling  point  on  the  air 
thermometer.  By  extrapolation  up  to  1000°  the  points  of  fusion 
of  gold  and  of  silver  were  found  quite  near  to  those  determined 
by  other  observers. 

Harker,  working  at  the  National  Physical  Laboratory,  Eng- 
land, has  compared  the  readings  of  platinum  thermometers,  when 
reduced  to  the  gas  scale  by  the  use  of  Callendar's  difference  for- 
mula, with  the  readings  of  thermocouples  calibrated  at  the  Reich- 
sanstalt,  and  with  the  indications  of  an  inglazed  porcelain-bulb 
nitrogen  thermometer  at  contant  volume  of  the  Reichsanstalt 
form.  Specially  constructed,  compensated  electric  furnaces  were 
used  for  heating. 

As  shown  by  the  accompanying  table,  taken  from  one  series  of 
Barker's  measurements,  the  agreement  between  the  scales  of  the 
platinum-resistance  and  thermoelectric  pyrometers  was  within 
0.5°  C.  throughout  the  temperature  range  up  to  1000°,  although 
the  gas  pyrometer  gave  somewhat  discordant  results. 

COMPARISON    OF    PYROMETRIC   SCALES   BY   HARKER. 


Temperature. 

Gas  ther- 

Thermo- 

Pt ther- 

G-Pt. 

G-Th. 

P-Th. 

mometer. 

couple. 

mometer. 

523-I 

524.3 

524.39 

-i-3 

—  1.2 

-O.I 

598.5 

597-8 

597.62 

+0.9 

+0.7 

-0.  2 

641.1 

641  .  1 

641  -  75 

+0.6 

+O.O 

-0.6 

776.7 

775-5 

775.13 

+  1.6 

+  1.2 

-0.4 

820.0 

818.4 

818.31 

+  1-7 

+  1.6 

—  o.  i 

875-0 

875-4 

875.24 

—  O.2 

-0.4 

—  O.  2 

959-8 

956.0 

955-47 

+4-3 

+3-8 

-0.5 

1005.0 

1004.4 

1004.37 

+0.6 

+0.6 

—  o.o 

I 

A  very  careful  direct  comparison  of  the  reduced  indications 
of  several  platinum  thermometers  with  the  gas  scale  as  furnished 
by  the  constant-volume  nitrogen  thermometer  has  also  been 
made  by  Chappuis  and  Harker  at  the  International  Bureau  at 


ELECTRICAL  RESISTANCE  PYROMETER  199 

Sevres,  and  their  results  give  further  evidence  that  the  indications 
of  the  platinum  thermometer  up  to  600°  C.  can  be  sufficiently 
well  expressed  by  Callendar's  formula. 

There  is  another  method  of  comparison  of  temperature  scales 
which  is  capable  of  great  accuracy,  namely,  the  determination  on 
the  several  scales  of  the  freezing  and  boiling  points  of  a  number 
sof  pure  substances.  This  method  has  some  decided  advantages 
over  the  above  method  of  comparison  even  in  a  most  carefully 
compensated  electric  furnace.  Heycock  and  Neville  in  England, 
and  more  recently  Waidner  and  Burgess  at  the  Bureau  of 
Standards,  have  determined  the  freezing  points  of  several  pure 
metals  in  terms  of  the  scale  of  the  platinum  thermometer  stand- 
ardized at  o°,  100°,  and  444.70°  C.  (the  boiling  point  of  sulphur), 
and  find  that  the  freezing  points  so  determined  give  tempera- 
tures on  the  gas  scale  as  closely  as  the  latter  can  be  reproduced, 
as  shown  in  the  following  table: 

GAS  AND  RESISTANCE  TEMPERATURE  SCALES: 

Gas  scale.  Resistance  scale. 

Holborn  and  Day  and        Heycock  and        Waidner  and 

Day.  Sosman.  Neville.  Burgess. 


Cd 

321  .7 

320.0 

320.7 

321  .0 

Zn 

410  o 

418.2 

419.4 

419.4 

Sb 

630.6 

629.2 

630  .  i 

630.7 

Al 

657  .0 

658.0 

658.0 

Ag 

061   ^ 

960.0 

961  .9 

960.9 

Cu.. 

1084  .  i 

1082.6 

1082.0 

1083.0 

These  results  confirm  the  view  of  the  sufficiency  of  the 
Callendar  difference  formula  for  the  most  accurate  work  up 
to  the  upper  limit  of  the  safe  use  of  the  platinum-resistance 
thermometer. 

Holborn  and  Wien  have  shown  that  at  very  high  temperatures 
the  interpolation  formula  is  certainly  inexact.  The  resistance 
seems  to  become  asymptotic  to  a  straight  line,  while  the  formula 
leads  to  a  maximum  evidently  inacceptable ;  in  their  opinion  it 
would  be  better  represented  by  an  expression  of  the  form 

R.t  =  a  +  b(t  +  273)-. 


200  HIGH  TEMPERATURES 

Here  are  the  results  of  two  series  of  their  experiments  made  on 
the  same  wire: 

t                                                     R  t                                                       R 

Degrees.                                            Ohms.  Degrees.                                             Ohms. 

o 0.0355  o 0.0356 

1045 1510     1040 1487 

H93 1595    "44 1574 

1303 1699    1328 1720 

1395 T787    1425 1802 

1513 1877    1550 1908 

1578 1933    1610 1962 

Using  the  Callendar  formula  and  platinum  wires,  Petavel 
found  the  melting  point  of  palladium  to  be  1489°,  which  Callendar 
and  Eumorfopoulos  found  to  be  1550°.  This  latter  number  is 
in  exact  agreement  with  the  best  determinations  of  this  tem- 
perature. 

Although  the  work  of  Holborn  and  Wien,  as  well  as  that  of 
Tory  and  others,  shows  that  the  platinum-resistance  thermometer 
made  of  fine  wire  cannot  be  depended  upon  to  remain  constant 
above  1000°  C.,  yet,  in  the  range  from  —200°  C.  to  +1000°  C.,  it 
serves  as  the  most  accurate,  and,  on  the  whole,  most  convenient 
method  of  measuring  temperatures  where  great  precision  is 
required,  and  is  particularly  adapted  for  the  delicate  control  of 
a  given  temperature. 

Dickson  has  proposed  the  formula 

(R  +  a)*  =  p(t  +  b), 

in  which  #,  6,  and  p  are  constants.  It  possesses  the  possible 
theoretical  advantage  over  the  Callendar  formula  of  not  requir- 
ing a  maximum  value  for  the  resistance  of  platinum.  This  form, 
however,  does  not  lend  itself  to  the  convenient  graphical  treat- 
ment applicable  to  the  difference  formula;  and  furthermore,  for 
thermometers  of  pure  platinum  calibrated  at  three  temperatures 
in  the  usual  way,  the  Dickson  formula  does  not  reproduce  the 
same  temperature  scale  as  the  difference  formula  as  shown  by 
Waidner  and  Burgess,  it  giving,  for  instance,  1051°  C.  for 


ELECTRICAL   RESISTANCE   PYROMETER  2OI 

copper  instead  of  1083°  C.  for  calibration  in  ice,  steam,  and 
sulphur  vapor. 

Nomenclature.  —  To  determine  a  temperature  by  means  of  a 
platinum  thermometer,  if  the  instrument  has  not  been  calibrated 
already  in  degrees,  it  is  necessary  to  know  the  difference  coefficient 
5  of  the  wire,  which  may  be  obtained  by  finding  the  platinum 
temperature  pt  at  some  known  point,  as  the  sulphur  boiling  point 
(S.B.P.),  or  by  comparison  with  a  calibrated  instrument. 

Callendar  has  suggested  the  following  notation  which  seems 
convenient  for  platinum  thermometry: 

Fundamental  Interval.  —  The  denominator  RIQQ  —  RQ  in  the 

formula 

IPO  (R  -  RQ)  (  , 

Pt  =  ~T£~    ~j?V>    ......     W 

^AIOO  —  HQ) 

for  the  platinum  temperature  pt,  represents  the  change  of  resist- 
ance of  the  thermometer  between  o°  and  100°. 

Fundamental  coefficient  =  c  =  mean  value  of  temperature  co- 
efficient of  change  of  resistance  between  o°  and  100°: 

_  (RiQQ  —  RQ) 
IOO  RQ 

Fundamental   zero  =  ptQ  =  -  =  reciprocal  of  fundamental  co- 

c 

efficient.  It  represents  the  temperature  on  the  scale  of  the 
instrument  itself  at  which  its  resistance  would  vanish. 

Difference  Formula.  —  The  following  form  is  the  most  con- 
venient for  computation: 

—  -iV  —  .....     (2) 

IOO          /      IOO 

Parabolic  function  expresses  the  vanishing  at  o°  and  100°  of 
above  formula,  which  becomes 


"  S.B.P.  "  Method  of  Reduction.  —  D  is  obtained  very  con- 
veniently by  determining  R",  and  thus  pt"  at  l"  =  the  boiling 
point  of  sulphur  (=  S.B.P.). 


202  HIGH  TEMPERATURES 

Resistance  Formula.  —  The  parabolic  difference  formula  is 
equivalent  to  assuming 

R_..I+at  +  bt2  () 

where 

/          d  \      ,  cd. 

a=c(i-\ ),    o= > 

\        loo/  10,000 

or  6  =  -  -   ' ' I0       • 

a  +  o  •  ioj 

Graphic  Method  of  Reduction.  —  An  easy  way  to  reduce  plati- 
num temperatures  to  the  gas  scale  is  to  plot  the  difference  t  —  pt 
in  terms  of  t  as  abscissas,  and  to  deduce  graphically  the  curve  of 
difference  in  terms  of  pt  as  abscissas.  This  is  most  convenient 
for  a  single  instrument  up  to  500°. 

Other  methods  have  been  used  by  Heycock  and  Neville  and 
by  Tory. 

Difference  Formula  in  Terms  of  pt.— 


This  formula  is  to  be  used  only  where  a  high  degree  of  accuracy 
is  not  required.  The  value  of  d'  may  be  determined  from  S.B.P., 
or  approximately 

d 


d'  = 


(i  —  0.077 


Construction  of  the  Platinum  Thermometer.  —  Callendar  first 
devised  a  satisfactory  and  perhaps  the  most  commonly  used  form 
of  platinum  thermometer,  in  which  the  platinum  wire  is  wound 
on  two  strips  of  mica  set  crosswise.  In  Fig.  61  is  shown  a  labora- 
tory form  of  Calendar's  potential  terminal  thermometer  used 
at  the  Bureau  of  Standards  in  precision  work  to  1100°  C.  The 
heavy  copper  head  insures  a  minimum  of  thermoelectric  effects 
at  the  platinum-copper  junctions,  and  provision  is  made  for  air 
cooling  of  the  head,  which  is  an  advantage  for  work  at  the  highest 
temperatures.  The  junctions  of  the  leads  to  the  platinum  coil 


ELECTRICAL  RESISTANCE  PYROMETER 


203 


are  easily  made  by  arc  soldering,  using  platinum  as  one  terminal 
and  a  graphite  pencil  as  the  other.  No  material  other  than 
platinum  should  enter  into  joints  to  be 
heated.  Forms  of  mica  supporting  frame 
are  shown  in  Fig.  62. 


,,-Air 
Circulation 


Porcelain 
Tube 


>  Mica  Disks 


Serrated  Mica  Frame 


Mica  Frame 


Fig.  62.     Mica  Frames. 


Various  modifications  of  the  above  de- 
sign are  used  in  the  industrial  forms, 
they  being  in  general  so  arranged  as  to 
secure  the  maximum  of  protection  and 
robustness  to  the  platinum  coil.  Mica 
frames  are  sometimes  replaced  by  steatite 
by  Leeds  and  Northrup  except  for  the 
highest  temperatures.  Industrial  types 
of  mounting  used  by  the  Cambridge 
Company  are  shown  in  Fig.  63. 

The  outer  containing  tubes  for  indus- 
trial instruments  are  preferably  of  metal, 
0  such  as  nickel  or  iron,  over  a  quartz 

Fig.  61.     Resistance  Py-  n      • 

rometer,  Laboratory  Type,  or  porcelain  tube,  the  actual  material  of 
the  sheath  depending,  however,  on  the  use  to  which  it  is- to- 
be  put. 


204 


HIGH  TEMPERATURES 


Platinum  Coil 


Screwed  Flange 


Porcelain  Tube-- 

Mica  Discs/" 

4  Platinum  Leads- 


SUPERHEATER 
THERMOMETER. 


K35mmH 

THERMOMETER  FOR  FLUE 
TEMPERATURES. 


Fig.  63.    Types  of  Industrial  Mounting. 


ELECTRICAL   RESISTANCE  PYROMETER 


205 


Porcelain, 
Tubes 

Steatite 
Disk 


Mica 
Partition 


Platinum 
-Coil 


Steatite 


Fig.  64.     Freely  Sus- 
pended Coil. 


For  use  at  very  high  temperatures,  Leeds  and  Northrup  have 

designed  the  form  of  potential  lead  thermometer  shown  in  Fig.  64. 

Heavy  wire  (0.6  mm.)  is  used  in  the  coil, 

which  is  freely  suspended  and  therefore  not 

subject  to  strains  on  cooling.  Due  to  its  very 

low  resistance,  special  precautions  have  to 

be  taken  in  the  temperature  measurements 

to   secure   sensibility.     Such   heavy-wire 

thermometers  will  change  their  constants 

very  much   less  than  those  of    fine  wire 

when  heated  to  high  temperatures.  Thus 
Waidner  and  Burgess  found 
I  that  heating  them  for  several 
hours  to  1200°  or  1300°  C. 
changed  the  zero  reading  by 
only  a  few  tenths  degree,  after  they  had  been 
once  annealed  at  1300°  C. 

In  order  to  secure  an  instrument  of  small  vol- 
ume and  at  the  same  time  satisfactorily  protect 
and  rigidly  mount  the  platinum  coil,  Heraeus  has 
devised  the  form  shown  in  Fig.  65,  in  which  the 
platinum  coil  is  embedded  in  fused  quartz  glass. 
The  behavior  of  this  type  of  thermometer,  with 
wires  of  0.05  to  0.15  mm.,  has  been  studied  at  the 
Reichsanstalt.  The  effect  of  embedding  in  quartz 
is  to  decrease  the  value  of  a  (equation  (3),  page  202) 
and  increase  the  value  of  5.  As  compared  with 
wires  mounted  in  the  usual  way,  and  receiving 
the  same  heat  treatment,  the  change  in  the  con- 
stants is  very  great  for  these  thermometers.  For 
the  former,  a  decreased  by  0.45  per  cent  and  8  by 
0.65  per  cent;  for  the  latter,  the  changes  were  1.7 
per  cent  and  6.7  per  cent  respectively. 

Where  very  great  rapidity  of  action  is  desired 
the  form  of  construction  shown  in  Fig.  66,  due  to 

Dickinson,  may  be  used  in  certain  cases,  the  metallic  parts  being 


Fig.  65. 

Mountings  in 

Quartz. 


206 


HIGH  TEMPERATURES 


preferably  all  of  platinum  where  great  permanence  is  desired,  and 
the  insulation  of  mica  strips. 

Where  platinum  thermometers  are  to  be  used  with  a  definite 
form  of  measuring  apparatus,  or  where  several  such  thermometers 

are  to  be  used  with  a  single  bridge, 
recorder,  or  other  indicating  or  reg- 
istering device,  it  is  convenient 
to  have  them  all  adjusted  to  exactly 
the  same  resistance  at  zero  and 
of  the  same  fundamental  interval, 
and  so  make  them  interchangeable. 
This  is  done  by  several  firms  by 
means  of  auxiliary  manganin  coils 
set  into  the  thermometer  head. 

Choice  of  Size  of  Wire.  —  Re- 
garding the  choice  of  the  diameter 
of  wire  to  use  in  constructing  a 
thermometer  coil  of  given  resist- 
ance, there  are  several  points  to 
consider  besides  the  current-carry- 
ing capacity  without  undue  heat- 
ing of  the  coil,  which  is  in  favor  of 
the  heavy  wire ;  such  as  the  greater 
temperature  lag,  heat  conduction 
along  the  leads,  and  excessive  size 
of  the  thermometer  coil,  which,  together  with  the  cost,  are 
the  main  inconveniences  of  heavy  wire;  and  liability  to  strains, 
fragility,  and  greater  evaporation,  which  limit  the  use  and  pre- 
cision of  too  small  wire.  It  is  easy  to  get  enough  current  sen- 
sibility, constancy  of  resistance,  and  robustness  with  wires  of  0.15 
to  0.20  mm.  diameter  except  for  very  low-resistance  pyrom- 
eters, 2  ohms  or  less,  which  are  to  be  avoided,  save  for  work  at 
very  high  temperatures,  as  taxing  too  severely  the  sensitiveness 
of  ordinary  forms  of  measuring  apparatus. 

Precautions  in  Construction  and  Use.  —  The  platinum  ther- 
mometer, as  usually  constructed,  is  a  fragile  instrument  in  spite 


Fig.  66. 


Thermometer  of  Small 
Lag. 


ELECTRICAL  RESISTANCE   PYROMETER  207 

of  its  appearance  of  robustness  when  encased  in  a  metal  tube, 
therefore  careful  handling  is  required.  To  avoid  breaking  from 
sudden  heating  when  porcelain  or  similar  containing  tubes  are 
used,  the  pyrometer  should  be  installed  in  advance  in  the  furnace, 
or  preheated  in  a  muffle  if  it  is  necessary  to  introduce  it  into  the 
hot  furnace.  It  is  necessary,  also,  to  heat  a  sufficient  length  of 
the  stem  in  the  furnace  in  order  to  avoid  the  effect  of  heat 
conductivity,  which  would  prevent  the  thermometer  spiral  from 
taking  up  the  temperature  of  the  space  in  which  it  is  immersed. 
Platinum  is  readily  attacked  and  its  resistance  changed  by  con- 
tact with  most  substances,  including  many  vapors  and  gases,  so 
that  the  thermometer  coil  must  be  carefully  shielded  by  materials 
impervious  to  the  atmosphere  in  which  it  is  placed,  such  as 
porcelain  glazed  on  the  outside.  As  platinum  changes  its  nature 
with  heating,  and  as  the  frame  on  which  the  coil  is  wound  may 
permanently  change  its  dimensions,  especially  if  mica  is  used, 
the  thermometer  before  calibration  should  be  annealed  at  a  tem- 
perature higher  than  that  at  which  it  is  to  be  used.  A  plati- 
num thermometer  will  change  its  readings  with  time  the  more 
rapidly,  the  higher  the  temperatures  at  which  it  is  used ;  therefore, 
in  order  to  control  its  constancy,  it  is  necessary  to  take  its  reading 
occasionally  at  some  known  temperature,  as  the  ice  or  steam  point. 
Well-shielded,  pure  platinum  wound  on  a  frame  that  does  not 
contaminate  the  wire  will  change  its  constants  with  use  less  than 
does  impure  platinum,  so  that  it  is  highly  desirable  to  use  only 
the  purest  of  platinum  in  the  construction  of  pyrometers.  Even 
with  pure  platinum,  however,  in  work  of  great  precision,  it  is 
necessary  to  recalibrate  occasionally,  and  when  temperatures 
above  1000°  C.  are  measured  frequently  this  operation  becomes 
very  onerous.  Great  care  has  to  be  exercised,  and  this  should  be 
especially  emphasized  for  industrial  as  well  as  scientific  installa- 
tions, to  secure  a  proper  insulation  of  all  electrical  circuits. 

Methods  of  Measurement.  —  It  is  evident  that  most  of  the 
ordinary  methods  for  the  measurement  of  resistance  may  be 
used  in  platinum  thermometry,  but  in  practice  only  a  few  of 
these  methods  have  been  applied  to  temperature  measurements, 


208  HIGH  TEMPERATURES 

although  there  is  a  tendency  at  the  present  time,  in  the  solution 
of  specific-temperature  problems,  to  take  advantage  of  the  pecu- 
liarities of  less  usual  methods  both  for  work  of  high  precision  in 
the  laboratory  and  for  industrial  applications.  Thus,  in  addition 
to  the  ordinary  slide- wire  and  dial  Wheats  tone  bridge  methods, 
the  Kelvin  double  bridge  is  sometimes  used  with  pyrometers  of 
very  low  resistance,  for  which  this  method  is  particularly  adapted. 
Potential  terminal  and  differential  galvanometer  methods  are 
also  used  in  precision  work,  and  for  industrial  practice  several 
deflection  methods  have  been  developed  for  the  direct  reading  of 
temperatures  on  a  galvanometer  scale. 

Compensation  for  Pyrometer  Leads.  —  There  is  one  character- 
istic in  the  measurement  of  a  resistance  coil  used  as  a  pyrometer 
that  distinguishes  it  from  an  ordinary  resistance  measurement, 
namely,  that  in  the  case  of  the  pyrometer  coil  there  is  a  region 
of  great  temperature  gradient  from  the  coil  to  the  measuring 
apparatus,  so  that  it  becomes  imperative  to  eliminate  the  variable 
resistance  of  the  leads  to  the  pyrometer  coil  —  a  resistance  that 
varies  both  with  the  depth  of  immersion  of  the  coil  and  with  its 
temperature.  There  are  several  ways  of  effecting  the  necessary 
compensation  of  this  variable  lead  resistance,  and  they  will  be 
described  under  the  several  headings. 

Three-lead  Thermometer.  —  This  was  the  form  originally  given 
to  the  instrument  by  Siemens  in  1871,  and  it  is  used  in  the  con- 
struction of  apparatus  suitable  for  industrial  use  by  Siemens  and 
Halske  and  by  Leeds  and  Northrup. 

In  the  Siemens  method  (Fig.  67),  the  thermometer  coil  P  forms 
one  arm  of  a  Wheatstone  bridge,  of  which  the  others  are  r\,  r2, 
and  R,  when  from  the  principle  of  the  bridge,  if  the  galvanometer 

G  remains  undeflected,  P  =  R— ,  neglecting  the  leads. 

fi 

The  compensation  for  the  variable  resistance  of  the  thermom- 
eter leads  is  effected  in  the  following  manner:  The  lead  aa',  of 
the  same  material  as  the  thermometer  coil  P  to  avoid  thermo- 
electric effects  at  their  junction,  is  constructed  to  be  as  exactly 
equal  as  possible  electrically  to  the  similar  lead  bb'.  The  lead 


ELECTRICAL   RESISTANCE   PYROMETER 


2OQ 


ad  is  in  the  P  arm  of  the  bridge  and  the  lead  W  is  put  in  the  R 
arm  by  means  of  the  auxiliary  lead  c'b  of  the  same  material  as  P. 


a  a' 

Fig.  67.     Three-lead  Compensated  Thermometer. 

This  lead  c'b  may  be  put  in  the  battery  circuit  as  shown,  or  in 
the  galvanometer  circuit  if  preferred.  It  is  not 
necessary  to  adjust  c'b  to  any  particular  resist- 
ance, so  that  fine  wire  may  be  used  for  it.  With 
this  arrangement,  therefore,  the  resistance  of  the 
thermometer  remains  apparently  constant  for  a 
given  temperature  whatever  its  depth  of  immer- 
sion and  whatever  the  temperature  gradient  along 
the  leads  aa',  bb',  so  long  as  it  is  the  same  for 
both. 

The  three-lead  compensated  thermometer  may 
also  be  used  with  a  differential  galvanometer. 
Fig.  68  shows  the  principle  of  such  an  arrange- 
ment for  an  instrument  of  Leeds  and  Northrup. 
The  slider  d  is  set  on  the  slide  wire  i  in  such  a  Use  of  Differential 
position  that  the  current  from  B  divides  equally  Galvanometer, 
between  the  circuits  b  +  R  +  gi  and  T  +  a  +  #2,  of  which  g\  and 
gz  are  the  two  differential  galvanometer  coils.  If  the  resistance 


Fig.  68. 


210 


HIGH  TEMPERATURES 


R  remains  fixed,  the  changes  in  temperature  of  T,  the  ther- 
mometer coil,  may  be  read  directly  in  degrees  on  the  slide  wire 
if  desired.  The  compensation  by  means  of  the  leads  #,  b,  c  is 


Fig.  69.     Thermometer  of  Siemens  and  Halske. 

effected  as  before.  The  arrangement  used  by  Siemens  and 
Halske  is  shown  in  Fig.  69.  In  Fig.  70  is  shown  a  system  of 
wiring  for  four  thermometers  of  the  Siemens  type  and  used 
with  a  single  indicator. 

For  work  of  great  precision,  this  method  is  of  course  capable 


ELECTRICAL  RESISTANCE   PYROMETER 


211 


of  elaboration  and  refinements,  as  in  the  calorimetric  measure- 
ments of  Jager  and  Steinwehr,  who,  however,  used  a  four-lead 
thermometer. 

Four-lead  Thermometer.  —  There  are  four  ways  in  which  the 
four-lead  compensated  thermometer  of  Callendar  and  Griffiths 


Fig.  70.     Four  Thermometers  with  One  Indicator. 

has  been  used,  namely,  the  Wheatstone  and  Kelvin '  bridge,  the 
potential  terminal,  and  the  differential  galvanometer  methods. 
The  Wheatstone  bridge  method  is  illustrated  in  Fig.  71,  from 
which  it  is  seen  that  the  compensating  leads  are  inserted  in  one 
arm  R  of  the  bridge  and  the  thermometer  leads  in  the  other. 
It  is  necessary  that  all  four  leads  be  as  nearly  as  possible  of  the 
same  length,  diameter,  and  material.  For  work  of  great  accuracy 
it  is  necessary  to  take  all  the  precautions  which  obtain  in  exact 


212 


HIGH  TEMPERATURES 


resistance  measurements,  and  in  particular  the  elimination  of 
thermoelectric  effects  and  uncertainties  in  the  exact  value  of  the 
ratio  coils. 

Precision  Bridges.  —  In  Fig.  72  is  shown  diagrammatically  the 
important  features  of  a  bridge  designed  and  in  use  at  the  Bureau 
of  Standards,  constructed  by  Leeds  and  Northrup,  and  capable 
of  measurements  to  i  in  100,000,  and  connected,  in  the  figure, 
for  use  with  a  four-lead  thermometer.  This  bridge  can  also  be 
used,  however,  with  a  three-lead  thermometer.  Some  of  its  char- 


Compensating  Leads 
Coil  Leads.  V 


Fig.  71.     Four- lead  Compensated  Thermometer. 

acteristics  are  possibility  of  reversal  of  all  circuits,  the  inter- 
changeability  of  the  ratio  coils,  mercury  contacts  for  the  higher 
resistances  to  eliminate  contact  resistances,  a  device  due  to 
Waidner  consisting  in  a  split  ohm  shunted  across  three  dials  to 
give  rapidity  of  setting  for  final  adjustment,  and  the  ability 
to  test  the  bridge  without  other  accessories.  The  bridge  is  oil- 
immersed  and  kept  at  constant  temperature  by  thermostatic 
control,  and  all  coils  are  of  seasoned  manganin,  which  for  the 
very  highest  precision  should  be  sealed  air-tight  separately  to 
avoid  effects  of  humidity  even  beneath  the  oil.  As  galvanometer, 
a  very  sensitive  form  of  d'Arsonval,  due  to  Weston,  is  used,  and 


ELECTRICAL  RESISTANCE   PYROMETER 

as  battery  one  to  three  dry  cells.  The  thermoelectric  key  may 
be  dispensed  with  and  a  single  contact  key  put  in  the  battery 
circuit,  with  a  variable  resistance  to  replace  the  usual  galvanom- 
eter shunt  for  varying  the  sensibility. 

Another  form  of  the  Callendar  and  Griffiths  self-testing  bridge, 
designed  primarily  for  use  with  resistance  thermometers  of  a 
fundamental  interval  of  one  ohm,  is  constructed  by  the  Cam- 
bridge Scientific  Instrument  Company  To  eliminate  tempera- 


Griffith's  Thermoelectric  Key- 
Oil  Immersed 


X  0.001  X  0.0001  X  0.00001 


Fig.  72.     Thermometer  Bridge  of  the  Bureau  of  Standards. 

ture  variations  in  this  bridge,  not  only  the  coils  but  the  bridge 
wire  and  all  contacts  are  oil-immersed;  and  it  is  capable  of 
reading  platinum  temperatures,  in  the  latest  model,  to  better 
than  0.01°  C.  by  direct  reading  on  the  scale  of  the  bridge  wire, 
when  a  galvanometer  of  suitable  sensibility  and  resistance  is  used, 
such  as  a  Broca  instrument  of  10  ohms. 

The  principle  of  the  construction  and  wiring  of  this  bridge  is 
shown  in  Fig.  73,  in  which  RI  and  R%  are  ratio  coils  of  10  co  each, 
which  should  be  interchangeable,  BC  the  balance  arm,  adjustable 
by  nine  manganin  coils  AR  and  the  slide  wire  s,  while  DC  is  the 


214  HIGH  TEMPERATURES 

thermometer  arm.     P  and  C  are  the  thermometer  and  compen- 
sating leads  respectively. 

The  unit  of  the  bridge  is  one  degree  on  the  platinum  scale 
(page  201),  and  this  corresponds  to  o.oi  co  for  a  FJ.  of  i  co 
in  the  thermometer.  This  bridge  possesses  many  mechanical 
excellencies,  such  as  a  special  form  of  combined  plug  and 
mercury  contact,  protection  from  mercury,  and  a  convenient 
form  of  vernier  and  slide  wire. 


Fig.  73.     Callendar  and  Griffiths  Bridge. 


The  resistance  of  the  potential  terminal  thermometer  is  deter- 
mined by  sending  the  same  current  from  a  storage  battery 
through  the  thermometer  and  a  known  resistance  in  series,  and 
measuring  the  potential  drop  by  means  of  a  potentiometer 
(page  138),  first  across  the  known  resistance  and  then  across 
the  thermometer  coil.  This  method  of  measurement  for  accurate 
•work  is  illustrated  in  Fig.  74,  which  shows  a  rheostat  and  milli- 


ELECTRICAL   RESISTANCE   PYROMETER 


215 


ammeter  in  the  circuit  for  adjusting  the  measuring  current.  The 
mercury-contact  resistance  box  may  be  adjusted  to  within  o.oi 
ohm  of  the  thermometer,  thus  eliminating  potentiometer  errors. 
This  box  may  of  course  be  replaced  by  a  single-standard  resist- 
ance, in  which  case  an  accurate  calibration  of  the  potentiometer 
is  required. 

The  current  leads  for  this  type  of  thermometer  do  not 
have  to  be  adjusted  to  equality,  and  the  potential  leads  may 
be  of  fine  wire,  as  may  also  the  current  leads,  but  still 
keeping  the  thermometer  sufficiently  robust,  so  that  errors 


To  Potentiometer 


To  Potentiometer- 


Milli  Ammeter 


A 


Rheostat 


Mercury  Contact  Resistance  Box 
.01     .01     .02    .02    .05      .1      .2  25 


Current  Leads 
Therm.  Coil  \  \   N 


Fig.  74.     Potential  Terminal  Thermometer  of  Precision. 


due  to  heat  conduction  along  the  leads  need  not  creep  into 
the  measurements. 

The  Kelvin  Bridge.  —  The  principle  of  this  method  of  measur- 
ing resistances  is  shown  in  Fig.  75,  in  which  61  is  an  adjustable 
resistance,  x  the  unknown,  and  the  others  are  such  that,  by 

construction,  7  =  7^,  when  x  =  -S  for  no  current  in  the   eral- 
b      0i  o 

vanometer.  This  method  of  bridge  design,  accompanied  by 
a  sufficiently  sensitive  galvanometer,  permits  the  measurement  of 
o.oi  ohm  to  be  made  with  about  the  same  precision  as  100  ohms 
by  the  usual  bridge  methods,  and  is  therefore  particularly  well 
adapted  for  resistance  thermometers  which  are  to  be  used  at 
very  high  temperatures,  because  such  instruments  must  be  made 


2l6 


HIGH  TEMPERATURES 


of  wire  of  large  diameter,  and  therefore  of  low  resistance,  in  order 
to  avoid  changes  in  their  constants  due  to  heating.  The  Kelvin 
bridge  method  permits  cutting  down  the  amount  of  platinum 
in  the  pyrometer,  an  advantage  both  in  cost  and  in  volume  of  the 
instrument. 

Leeds  and  Northrup  made  a  potential  point  indicator  (Fig. 
75  A),  with  slide  wire  for  use  with  a  heavy-coil  low-resistance 
thermometer  carrying  a  current  of  0.3  ampere.  The  extension 
coils  and  slide  wire  may  be  graduated  in  degrees  of  tem- 
perature for  any  given  thermometer.  The  high  values  (520  «) 
of  a  and  a'  (Fig.  75),  necessary  to  eliminate  resistance  changes 
in  the  potential  'leads,  require  that  the  galvanometer  used 


Fig.  75.     Principle  of  Kelvin  Bridge. 

shall  have  a  greater  sensibility  than  can  easily  be  gotten 
in  a  portable  pointer  instrument.  The  type  of  galvanometer 
is  the  same  as  that  required  for  high-precision  Wheatstone 
bridge  work  with  proper  adjustment  of  critical  external  re- 
sistance. 

Sensibility.  —  The  sensitiveness  of  the  measurements  in  re- 
sistance thermometry  is  that  of  the  very  great  precision  attain- 
able in  resistance  measurements,  or  it  may  be  better  than  i  in 
100,000,  or  about  0.001°  C.  for  a  high-temperature  thermometer 
whose  resistance  at  o°  C.  is  from  3  to  25  ohms,  if  proper  pre- 
cautions are  taken.  The  factors  limiting  the  sensibility  of  re- 
sistance measurements  in  the  Wheatstone  bridge  method,  for 
example,  and  which  are  inherent  in  thermometric  work,  are  the 


ELECTRICAL  RESISTANCE   PYROMETER  217 

practical  necessity  of  using  a  i  :  i  ratio,  required  for  the  lead 
compensation;  the  need  of  keeping  the  current  through  the  ther- 
mometer coil  so  low  as  not  to  raise  its  temperature  unduly;  and 
finally,  the  sensibility  of  the  galvanometer.  Due  to  the  first  and 


P, 


\^) —   i OvwwOwvwO 


Fig.  75  A.     Potential  Point  Indicator. 

second  of  these  conditions,  the  ordinary  rules  for  the  Wheatstone 
bridge  do  not  apply  without  modification,  and  fortunately  the 
limitations  they  impose  may  very  largely  be  overcome  by  prop- 
erly choosing  the  constants  of  the  thermometer  and  galva- 
nometer. It  should  be  remarked  that,  with  the  d'Arsonval  or 


2l8  HIGH  TEMPERATURES 

moving-coil  galvanometers  of  very  great  sensibility  and  of  prac- 
tically constant  zero  which  are  available  to-day,  the  question  of 
the  realization  of  sufficient  sensibility  is  of  distinctly  secondary 
importance  in  accurate  work.  In  the  case  of  recording  instru- 
ments, when  in  general  a  less  sensitive  instrument  must  be  used, 
some  attention  has  to  be  paid  to  the  matter,  and  particular  care 
has  to  be  taken  here  to  so  arrange  as  not  to  overheat  the  ther- 
mometer with  the  larger  currents  required  by  such  galvanometers. 
For  the  maximum  current  through  the  galvanometer  and  the 
minimum  through  the  thermometer  coil,  with  a  battery  of  neg- 
ligible resistance,  the  bridge  should  be  arranged  as  follows,  as 
shown  by  Callendar:  "  Connect  the  battery  so  as  to  make  the 
resistance  in  series  with  the  thermometer  greater  than  the  resist- 
ance in  parallel." 

Direct-reading  Thermometers.  —  There  have  been  in  recent 
years  a  considerable  number  of  direct-reading  resistance  py- 
rometers devised  by  several  manufacturers.  We  shall  be  able 
to  call  attention  to  only  a  few  typical  instruments,  which  are,  of 
course,  of  interest  mainly  in  technical  practice.  A  principle 
commonly  made  use  of  in  some  of  its  modifications  is  that  of 
the  ohmmeter,  in  which  a  variable  resistance,  that  of  the  ther- 
mometer, is  balanced  against  a  fixed  resistance  by  means  of  the 
deflection  of  a  galvanometer  coil  carrying  currents  from  circuits 
shunted  around  the  two  resistances  in  question.  Such  deflection 
instruments  are  constructed  by  Paul,  Hartmann  and  Braun, 
Carpentier,  Leeds  and  Northrup,  and  others. 

The  Harris  Direct-reading  Resistance-thermometer  Indicator, 
manufactured  by  Mr.  Robert  W.  Paul  of  London,  indicates  tem- 
peratures directly  by  the  movement  of  a  pointer  over  a  scale; 
moreover  its  accuracy  is  independent  of  the  battery  or  supply 
voltage  used. 

In  principle  it  is  a  two-coil  ohmmeter,  or  coil-controlled 
galvanometer;  the  requisite  sensitivity  to  the  small  changes 
in  resistance,  which  are  utilized  in  platinum  thermometry, 
is  attained  by  making  the  action  of  the  deflecting  kcoil 
differential. 


ELECTRICAL   RESISTANCE   PYROMETER 


219 


The  differential  windings  of  the  deflecting  coil  are  respectively 
connected  in  shunt  with  the  platinum  thermometer  and  a  resist- 
ance equivalent  to  that  of  the  thermometer  at  any  desired  tem- 
perature, dependent  upon  the  part  of  the  temperature  scale  at 
which  it  is  desired  to  work.  The  control  coil  of  the  ohmmeter 
system  is  connected  in  shunt  with  a  resistance  suitably  chosen 
to  give  the  required  sen- 
sitivity. These  combina- 
tions are  connected  in 
series.  Hence,  on  the 
passage  of  an  electric  cur- 
rent, the  forces  due  to  the 
windings  are  proportional 
to  the  resistances  they 
respectively  shunt. 

In  the  accompanying 
vector  diagram,  the  plat- 
inum thermometer  is 
assumed  to  have  a  Fun- 
damental Interval  of  one 
ohm  and  to  be  brought 
up  to  a  resistance  of  three 
ohms  at  o°  C.,  by  means 
of  a  resistance  which  has 
no  temperature  coefficient, 
suitably  introduced  into 
the  circuit.  This  enables 

the    thermometers    to    be 

made  electrically  interchangeable  with  each  other. 

Considering  first  the  case  of  an  ohmmeter  system  without 
the  differential  winding  of  the  deflecting  coil,  let  AB  represent  the 
controlling  force  of  the  ohmmeter  system  (proportional  to  the 
control-coil  shunt  referred  to  above,  and  in  this  instance  taken  as 
one  ohm);  and  let  AC  represent  the  deflecting  force  with  a  plati- 
num thermometer  of  one  ohm  Fundamental  Interval  at  o°  C. 
The  pointer  of  the  instrument  will  then  take  up  the  position  AF. 


AC-AE 


AC-AE 


76'     Vector  Diagram  for  Ohmmeter. 


220  HIGH  TEMPERATURES 

Suppose  the  temperature  of  the  thermometer  is  raised  to  100°  C.; 
the  deflecting  force  now  increases  by  the  amount  of  CC'  (pro- 
portional to  one  ohm)  and  becomes  equal  to  AC,  causing  the 
pointer  to  set  along  AG.  Similarly,  if  the  thermometer  drops  to 
-ioo°C.,  the  position  taken  up  by  the  pointer  is  along  AH, 
giving  an  angle  6  for  200°  C.  variation. 

If,  however,  the  deflecting  coil  be  wound  differentially,  and  a 
current  equal  to  that  producing  the  deflecting  force  AC  be  passed 
through  the  other  winding  in  such  a  direction  that  its  effort  is  in 
opposition  to  AC,  introducing  the  vector  AE,  the  initial  position 
of  the  pointer  will  be  along  AB,  and  a  variation  CC'  in  the  vector 
AC  gives  a  resultant  AC"  and  causes  the  pointer  to  set  along  the 
line  AJ.  Similarly,  should  AC  decrease  by  an  amount  equal  to 
CC",  the  resultant  deflection  is  on  the  opposite  side  of  the  initial 
position,  and  the  pointer  takes  up  the  position  AK,  giving  the 
large  angle  <f>  for  the  change  in  the  thermometer  resistance  equal 
to  that  which  only  gave  the  angle  6  with  a  nondifferential  ar- 
rangement. It  will  be  noted  that  AE  may  be  given  a  value 
equal  to  A  C  at  any  required  temperature  of  the  platinum  ther- 
mometer, the  position  of  the  pointer  at  such  temperature  lying 
along  AB  and  covering  the  same  angles  as  before  for  the  same 
ohmic  variation  in  the  thermometer  resistance.  By  simulta- 
neously varying  the  vector  AB  the  angle  <£  may  be  kept  true  to 
gas-scale  temperature.  It  is  thus  possible  to  construct  a  multiple- 
range  instrument. 

The  accompanying  diagram,  Fig.  77,  shows  the  scheme  of  con- 
nections for  such  an  instrument.  One  of  the  differential  windings 
(X  in  the  figure)  is  shunted  with  a  platinum  thermometer,  the 
other  winding  5  being  shunted  with  a  resistance  s,  which  is  made 
variable  so  as  to  equal  the  resistance  of  the  thermometer  at  certain 
fixed  temperatures.  The  control  coil  of  the  ohmmeter  system 
is  also  shunted  with  the  resistance  d,  the  value  of  which  is 
determined  by  the  degree  of  sensitiveness  required,  and  may  be 
made  variable  with  s.  These  shunted  windings  are  connected 
in  series,  and  the  circuit  is  completed  through  a  battery  and 
switch. 


ELECTRICAL   RESISTANCE   PYROMETER 


221 


In  this  arrangement,  since  the  currents  in  the  ohmmeter  system 
windings  depend  upon  the  resistance  of  the  platinum  thermom- 
eter, s  and  d  respectively,  the  value  of  s  may  be  taken  as  the 


Fig.  77.     Harris-Paul  Indicator. 

vector  AE,  increasing  by  steps  equal  to  the  rises  in  resistance  of 
the  platinum  thermometer  for  each  range  of  temperature.  The 
platinum  thermometer  represents  the  vector  AC,  AC,  etc., 
while  d  represents  AB.  This  is  made  variable  with  5  in  order 
that  the  instrument  shall  read  in  gas-scale  degrees  on  all  ranges, 
and  its  values  are  calculated  in  accordance  with  Callendar's 
formula  for  the  platinum  ther- 
mometer. 

The  Logometer  and  Ratiometer. 
—  Messrs.  Carpentier  and  Joly 
have  also  proposed  the  construc- 
tion of  a  deflectional-resistance 
pyrometer  based  on  the  use  of 
the  logometer,  an  apparatus  de- 
signed for  the  measurement  of 


Fig.  78.     Logometer  Coil. 


the  ratio  of  two  currents.  This  is  shown  in  plan  in  Fig.  78, 
where  two  oppositely  wound  coils,  similar  to  those  of  a  d'Arson- 
val  galvanometer,  are  mounted  by  double  pivot  in  the  unsym- 
metrical  field  of  a  permanent  magnet  NS.  If  the  two  coils 


222 


HIGH  TEMPERATURES 


have  the  same  number  of  turns,  we  have  iH  =  i'  Hf,  and  since 
the  electromagnetic  force  of  each  coil  is  directed  toward  a 
weaker  field,  the  final  position  of  the  coils  will  be  stable  and 
will  depend  only  on  the  ratio  of  the  two  currents  i  and  i'  in 
the  coils. 

For  the  measurement  of  resistance,  the  circuit,  in  one  of  its 
simplest  forms,  is  arranged  as  shown  in  Fig.  79,  the  logometer 
coils  being  shunted,  one  about  a  manganin  resistance  r  and  the 


Logometer 
Coil 


nrtnro 

innnr 

Manganin 
lAAAAAAAAAA 

Platinum 

IAAAAAAAA/V 

Thermometer 


Fig.  79.     Simple  Logometer  Circuit. 

other  about  the  platinum  thermometer  of  resistance  />;  when  if 

s  and  s'  are  the  coil  resistances,  their  currents  are  —  and  -^, 

s  s 

where  i  =  current  in  principal  circuit,  and  their  ratio  is  *-  •— , 

a  quantity  whose  variations  depend  only  on  p,  the  resistance 
of  the  platinum  thermometer,  neglecting  any  variations  in  the 
resistance  of  leads  to  the  logometer  coils.  The  logometer  dial, 
over  which  moves  a  pointer  attached  to  the  moving  coils,  may 
therefore  be  graduated  directly  in  degrees  of  temperature.  The 
readings  of  the  instrument  are  independent  of  the  value  or  of 
variations  in  the  current  i.  Measurements  may  be  taken  with 
alternating  currents,  and  when  the  manganin  and  platinum  coils 
are  noninductively  wound,  the  readings  will  be  independent  of 
changes  in  voltage  and  frequency.  This  instrument  may  be 
arranged  to  develop  relatively  powerful  directing  couples  and  is 
therefore  readily  rendered  recording. 


ELECTRICAL   RESISTANCE   PYROMETER 


225 


Northrup's  ratiometer,  similar  to  the  preceding,  is  also  an 
adaptation  of  the  deflection-ohmmeter  principle  to  temperature 
measurements.  Northrup  makes  use  of  the  three-lead  thermom- 
eter with  connections  as  shown  in  Fig.  80,  in  which  C\  and  C2 
are  two  flat  coils  mounted  on  a  damped  movable  system  between 
the  two  crescent-shaped  pole  pieces  of  a  permanent  magnet. 
This  instrument,  which  is  made  in  a  compact  form  and  read  by 
an  attached  microscope,  can  be  made  sensitive  to  about  0.1°  C. 
and  constant  to  better  than  2°  C. 


Fig.  80.     Northrup's  Ratiometer. 

The  Cambridge  Deflectional  Instrument.  —  The  Cambridge 
Scientific  Instrument  Company  also  have  recently  devised  a 
deflectional  method  for  the  measurement  of  temperature  with 
resistance  thermometers,  in  which  the  temperature  is  indicated 
by  the  "  out-of -balance  "  current  in  a  Wheats  tone  bridge,  pro- 
vided with  compensating  leads;  the  arms  of  the  bridge  are  all 
fixed  resistances  except  the  one  which  forms  the  resistance  ther- 
mometer. As  designed,  provision  is  made  for  exactly  setting 
the  zero  of  the  indicator  for  a  balance  of  the  bridge,  for  adjusting 
the  current  to  give  the  required  deflection  for  the  temperature 
range,  and  is  provided  with  an  "  ice  coil  "  for  balancing  the  ther- 
mometer at  o°  C. 


224 


HIGH  TEMPERATURES 


The  Whipple  indicator  shown  in  Fig.  81  is  a  dial  instrument 
using  the  Wheatstone  bridge  principle.  The  galvanometer  needle 
has  to  be  brought  to  rest  by  turning  a  knob,  when  the  dial  reading 
gives  temperatures  directly. 

The  Leeds  and  Northrup  Indicators.  —  This  firm  has  brought 
out  several  patterns  of  balance  and  deflection  indicator  instru- 
ments based  for  the  most  part  on  the  use  either  of  the  differential 
galvanometer  (page  209)  or  the  Kelvin  bridge  (page  215). 

A  very  convenient  deflection  indicator  with  adjustable  scale 
is  shown  in  Fig.  82.  The  dial  may  be  set  to  an}^  desired  tern- 


Fig.  8 1.     Whipple  Indicator. 

perature,  and  the  position  of  the  deflector  needle  indicates  how 
much  higher  or  lower  the  furnace  is  than  the  required  tempera- 
ture. The  deflector  has  a  very  open  scale,  permitting  readings 
to  be  taken  from  a  distance.  The  workman  has  to  concern 
himself  only  with  deflections  of  the  needle  from  the  vertical. 
The  accuracy  is  independent  of  voltage  fluctuations,  and  the 
instrument  may  be  run,  if  desired,  from  the  ordinary  lighting 
circuit.  Temperature  intervals  as  small  as  two  degrees  are 
readable. 

Calibration.  —  For  platinum  thermometers  which  are  to  be 
used  with  some  form  of  calibrated  resistance-measuring  appa- 
ratus, such  as  described  above,  it  is  only  necessary,  in  order 


ELECTRICAL   RESISTANCE   PYROMETER 


225 


to  calibrate  the  thermometer,  to  take  its  readings  at  three  tem- 
peratures, as  at  the  ice  point,  the  steam  point,  and  the  boiling 
point  of  sulphur,  when,  if  the  wire  is  of  pure  platinum,  the 
temperatures  found  by  using  Callendars  method  of  computation 
(see  page  201)  will  be  correct  to  as  close  as  they  are  known  in 
terms  of  the  gas  scale  to  1100°  C. 


Fig.  82.     Deflection  Indicator  of  Leeds  and  Northrup. 

There  is  advantage  in  using  a  fourth  calibration  point,  as  the 
silver  freezing  point,  or  that  of  Ag3  — Cu2,  in  calculating  the 
value  of  5  (page  201)  for  impure  wires  that  are  to  be  used  at 
high  temperatures.  For  the  whole  range  of  temperatures  with 
such  a  wire,  both  the  sulphur  and  silver  points  may  be  obtained, 
when  8  takes  the  form  a  +  bt. 

For  thermometers  to  be  used  with  direct-reading  temperature 
indicators,  it  is  necessary  to  compare  their  readings  with  those 


226  HIGH  TEMPERATURES 

of  a  standard  at  several  temperatures,  preferably  in  a  resistance 
furnace  of  the  Heraeus  type.  This  second  method  of  cali- 
bration is  usually  less  accurate  than  the  first.  Methods  of 
realizing  experimentally  the  sulphur  point  and  other  fixed 
temperatures  will  be  described  in  Chapter  XI  on  Standardi- 
zation. 

The  platinum  thermometer  may  be,  and  should  be,  for  techni- 
cal work,  so  constructed  as  to  read  directly  in  platinum  degrees, 
or  still  better  in  degrees  of  temperature.  This  method  saves 
much  time  and  chance  of  mistake.  The  calibration  curve, 
once  made  for  an  instrument,  serves  indefinitely,  with  occasional 
checking  up  if  used  at  high  temperatures;  so  that  in  spite  of  the 
appearance  of  complications  in  this  method  of  measuring  tem- 
peratures, actually  in  practical  use  the  determination  of  a  tem- 
perature on  the  normal  scale  by  the  platinum  thermometer  is 
the  affair  of  a  few  seconds  only. 

Reduction  Tables.  —  In  the  Appendix  are  given  tables  for  the 
reduction  of  platinum  temperatures  to  centigrade  temperatures 
for  wires  of  pure  platinum,  correction  tables  for  wires  of  impure 
platinum,  and  other  auxiliary  tables. 

Some  Results  Obtained.  —  There  is  a  remarkable  agreement 
among  the  fixed  points  obtained  by  several  observers  using  the 
platinum  thermometer,  for  observations  extending  over  twenty 
years,  as  shown  in  the  following  tables,  in  which  all  the  obser- 
vations were  obtained  by  calibrating  the  platinum  thermometer 
in  ice,  steam,  and  sulphur  vapor,  the  temperature  of  this  last 
being  here  taken  as  444. 70  on  the  constant-volume  nitrogen  scale, 
the  value  best  representing  the  work  of  these  observers  except 
Holborn  and  Henning. 

SCALE  OF  RESISTANCE   THERMOMETER. 

BOILING  POINTS. 
Naphthaline.     Benzophenone. 

Callendar  and  Griffiths  (1891) 217.97°  305.89° 

Travers  and  Gwyer  (1905) 218.07  305.87 

Holborn  and  Henning  (1908  and  1911)  * 217.96  305.89 

Waidner  and  Burgess  (1910) 217.98  306.02 

*  S.B.P.  =  444.51  C.  at  constant  volume. 


ELECTRICAL   RESISTANCE  PYROMETER  227 

SCALE  OF  RESISTANCE  THERMOMETER  (Continued) 

FREEZING  POINTS. 
Sn  Cd  Pb  Zn 

Callendar  and  Griffiths  (1891) 231.9  320.8  327.8  419.0 

Heycock  and  Neville  (1897) 231.9  419 .4 

Waidner  and  Burgess  (1909) 231.9  321.0  327.4  419.4 

Holborn  and  Henning  (1911) f   231.83  320.92         419.40 

FREEZING  POINTS. 
Sb  Al  Ag  Au  Cu 

Heycock  and  Neville 630.0          656*        961.9        1063.5        1082 

Waidner  and  Burgess 630. 7          658          960.9         1083 

*  Containing  0.5  per  cent  impurities.        t  S.B.P.  =  444. Si  C.at  constant  volume. 

Use  as  a  Standard.  —  In  1899,  Callendar,  at  a  meeting  of  the 
British  Association  for  the  Advancement  of  Science,  in  view  of  the 
relative  ease  and  great  precision  of  resistance  measurements  and 
the  great  difficulties  in  the  use  of  the  gas  thermometer,  suggested 
that  the  platinum  thermometer  be  adopted  as  a  secondary 
standard,  reducing  its  readings  as  above  indicated,  and  assuming 
as  calibration  points,  o°,  100°,  444.5°,  the  last  being  the  sulphur 
boiling  point  on  the  constant-pressure  scale.  All  platinum 
thermometers  could  then  be  compared  with  one  selected  as 
standard  and  calibrated  as  above  indicated.  He  also  pointed 
out  that  as  regards  portability  and  ease  of  reproduction,  it  is 
sufficient  to  send  a  few  grams  of  the  standard  wire  in  an  ordinary 
letter,  to  reproduce  the  scale  with  the  utmost  accuracy  in  any 
part  of  the  world. 

The  work  done  in  platinum  and  gas  thermometry  since  1899 
abundantly  justifies  Callendar's  suggestion  of  using  the  platinum 
thermometer  as  a  secondary  standard,  since,  as  has  been  shown  in 
preceding  paragraphs,  a  resistance  thermometer  of  pure  platinum 
calibrated  at  three  temperatures  reproduces  the  gas  scale  with 
the  greatest  exactness  to  as  high  temperatures  as  the  platinum 
thermometer  can  be  used  conveniently.  It  is  not  necessary,  how- 
ever, to  compare  a  platinum  thermometer  with  another  taken 
as  standard,  if  means  are  at  hand  for  an  independent  calibra- 
tion, since  the  characteristic  constants  of  pure  platinum  are 
now  known,  and  this  metal  can  easily  be  had  of  sufficient  purity 


228  HIGH  TEMPERATURES 

to  satisfy  them.  It  is  perhaps  better  to  take  the  sulphur  boiling 
temperature  on  the  constant-volume  scale,  as  most  of  the  recent 
work  in  the  determination  of  fixed  points  has  been  in  terms  of 
this  scale. 

For  a  platinum-resistance  thermometer  to  serve  as  a  secondary 
standard,  therefore,  provided  its  construction  and  use  are  other- 
wise correct,  it  is  necessary  and  sufficient  that  its  value  of  6  =  1.50 
(or  1.49  according  to  Holborn  and  Henning's  scale)  and  of 
C=  0.0039,  when  calibrated  in  ice,  steam,  and  sulphur  vapor. 
(See  chapter  on  Standardization.) 

Sources  of  Error  in  Accurate  Work.  —  Heating  by  the  Meas- 
uring Current.  —  It  is  evident  that  if  a  too  large  current  is  sent 
through  an  electrical  resistance  thermometer,  the  heating  thus 
occasioned  will  cause  the  indicated  temperatures  to  be  high. 
The  limiting  value  of  the  current  Callendar  has  shown  to  be 
about  o.oi  ampere  per  o.oi  degree  with  an  average  platinum 
thermometer  of  wire  0.15  mm.  in  diameter.  If  a  galvanometer 
of  sufficient  sensibility  is  used  this  effect  is  negligible,  and  when 
a  greater  current  has  to  be  used  on  account  of  lack  of  galva- 
nometer sensibility,  the  heating  effect  may  be  maintained  nearly 
constant  by  keeping  the  current  constant  by  means  of  a  rheostat 
in  the  battery  circuit,  since  the  resistance  of  the  thermometer 
increases  very  nearly  as  fast  as  the  rate  of  cooling,  or  a  little 
faster  than  the  temperature.  Callendar  also  indicates  that  the 
heating  effect  is  readily  measured  by  using  as  current  source 
two  storage  cells,  connected  first  in  parallel  and  then  in  series, 
the  current  heating  correction  being  given  by  subtracting  from 
the  first  reading  one-third  of  the  difference  between  the  two 
readings. 

Waidner  and  Eurgess  have  also  studied  the  heating  effect  of 
the  measuring  current  and  find  that  although  this  current  may 
heat  the  coil  to  more  than  i°  C.  above  its  surroundings,  the 
value  of  the  fundamental  interval  of  the  thermometer  remains 
the  same  as  when  a  current  one-fifth  as  great  is  used. 

The  effect  of  using  different  measuring  currents  with  a  ther- 
mometer of  RQ  =  348  co  is  shown  below: 


ELECTRICAL   RESISTANCE   PYROMETER  229 
HEATING  EFFECT  OF  MEASURING  CURRENT. 

Amperes.                       /?„                        P.I.                    #444.33                 Pts.B.f.  * 

2.5-IO-3            3.48160            I.34H5            9.13220            421-33  !-503 

io. o  •  io-3           3-48i74           I.34H3           9-J32I3           421.31  1-505 

50.0  •  io~3           3.48705           1.34114           9.13608           421.21  1.511 

loo.o-io-3          3-50373           I-34I73           9-I4832           420.69  1.545 

HEATING  OF  PT  COIL  ABOVE  SURROUNDING  TEMPERATURE. 

Amperes.  AT0  &Tm 


2.5    •  IQ— s  O.OOI  O.OOI  .... 

IO.O  •  ID" 3  .Oil  .OIO  .... 

5O.O   •  IQ— 3  .41  .41  .29 

IOO.O  •  IQ— 3  1.65  1.69  1. 2O 

For  a  given  small  excess  in  temperature  of  the  platinum  coil 
above  the  temperature  of  its  surroundings,  the  energy  radiated 
in  steam  is  3.7  and  in  sulphur  52  times  that  radiated  at  o°  C.r 
assuming  that  the  radiation  from  platinum  is  proportional  to  the 
fifth  power  of  the  absolute  temperature.  For  constant  measuring 
current,  the  energy  supplied  to  the  coil  at  the  S.B.P.  is  only  2.6 
times  the  energy  supplied  at  o°  C.  It  follows  that  the  greater 
part  of  the  energy  loss  is  by  convection  and  conduction  rather 
than  by  radiation. 

Lag  of  the  Platinum  Thermometer.  —  Inclosed  as  it  necessarily 
is  for  most  work  in  a  sheath  of  porcelain,  and  possessing  besides 
considerable  mass,  the  platinum  thermometer  does  not  immedi- 
ately assume  the  temperature  of  its  surroundings.  Put  into  a 
sulphur  bath,  it  assumes  an  equilibrium  condition  in  ten  minutes. 
For  small  changes  of  temperature  this  effect  is  hardly  perceptible 
and  may  be  neglected  in  most  practical  work. 

Inclosed  in  a  thin  flat-sided  metal  case  (see  Fig.  66),  the  tem- 
perature lag  is  practically  nothing. 

Insulation. —  Defective  insulation  due  to  moisture  condensed 
in  the  tubes  is  sometimes  a  source  of  error  in  accurate  work  at 
the  ice  point  and  lower  temperatures  with  thermometers  of  high 
resistance  if  the  tubes  are  not  sealed.  This  may  be  readily  done, 
if  the  containing  sheath  is  of  glass,  by  sealing  the  platinum  leads 
into  the  glass  so  that  they  terminate  in  cups.  When  the  con- 
taining sheath  is  of  porcelain,  as  for  high-temperature  work,  this 


230  HIGH  TEMPERATURES 

sealing  is  not  necessary,  nor  is  it  convenient;  but  running  the 
leads  into  metal  cups  containing  a  fusible  alloy  still  offers  the 
readiest  method  of  securing  a  good  contact  with  the  rest  of 
the  circuit. 

Compensation  for  Resistance  of  the  Leads.  —  It  is  necessary,  in 
order  to  avoid  thermal  currents  at  the  junctions  with  the  ther- 
mometer proper  and  also  evaporation  and  consequent  change  of 
resistance,  to  employ  platinum  leads  from  the  thermometer  to  a 
point  in  the  circuit  at  a  constant  temperature.  Even  if  these 
leads  are  of  relatively  large  diameter,  there  will  still  remain  an 
error  due  to  the  varying  resistance  of  these  leads  with  change  in 
temperature  and  with  varying  depth  of  immersion.  It  becomes 
necessary  either  to  apply  a  "stem  correction,"  which  is  trouble- 
some and  uncertain,  or  compensate  for  this  effect  as  described 
under  methods  of  measurement.  Nowadays  most  platinum  ther- 
mometers sold  for  industrial  and  scientific  purposes  are  compen- 
sated. Uncompensated  thermometers  with  gold  leads  are  also 
to  be  found.  They  are  not  to  be  recommended  for  work  of  high 
accuracy.  Silver  leads  are  to  be  avoided. 

The  copper  leads  from  the  thermometer  head  to  the  measuring 
apparatus  may  be  of  appreciable  resistance,  and  to  render  them 
flexible  they  are  often  stranded,  when  their  resistance  may  vary 
somewhat.  Thus  Mr.  F.  W.  Smith  has  found  copper  leads  of 
-^Q  co  resistance  to  vary  i  or  2  per  cent,  giving  0.003°  C.  uncer- 
tainty at  o°  C.  In  work  of  high  accuracy  it  is  evidently  as  im- 
portant to  keep  the  copper  leads  as  .constant  as  the  platinum. 
It  is  now  possible  to  obtain  stranded  wire  in  which  each  strand 
is  enameled,  and  so  eliminate  the  slip  resistance. 

In  addition  to  the  potential  lead  method,  there  have  been 
bridge  methods  devised  for  the  complete  experimental  elimina- 
tion of  all  leads  to  the  thermometer  which  is  required  in  work 
of  the  highest  accuracy,  as  it  is  extremely  difficult,  if  not  impos- 
sible, to  make  the  compensation  absolutely  exact  by  adjustment 
in  construction.  These  methods  require,  for  the  most  part, 
rather  elaborate  experimental  arrangements,  for  descriptions 
of  which  the  reader  is  referred  to  the  papers  of  Edwards, 


ELECTRICAL  RESISTANCE  PYROMETER  231 

W.  Jaeger,  and  F.  W.  Smith.  In  brief,  such  methods  depend 
either  upon  devices  for  alternately  throwing  the  thermometer 
leads  in  the  two  sides  of  the  bridge,  measuring  these  leads,  or 
eliminating  them  by  use  of  the  Kelvin  double  bridge  or  some 
modification. 

Conduction  along  the  Leads.  —  The  thermometer  leads  may  be 
the  seat  of  another  source  of  error,  which  increases  in  importance 
with  the  diameter  of  the  leads,  their  length  immersed,  and  the 
temperature  gradient,  namely,  the  effect  of  heat  conduction  along 
the  leads  influencing  the  resistance  of  the  thermometer  coil. 
This  effect  is  especially  to  be  looked  for  in  four-lead  bridge  ther- 
mometers, where  all  four  leads  are  of  relatively  heavy  platinum. 
The  best  way  to  eliminate  this  source  of  error  is  to  so  design  the 
instrument  that  it  is  negligible.  Its  presence  may  be  recognized 
and  corrected  for  by  varying  the  depth  of  immersion  of  the  ther- 
mometer in  a  bath  at  constant  temperature. 

Use  of  Impure  Platinum.  —  The  value  of  the  constant  5  in 
the  Callendar  formula  (2),  page  201,  is  a  measure  of  the  purity 
of  the  metal.  For  the  purest  platinum  the  value  of  5  is  1.500, 
assuming  the  S.B.P.  =  444.70,  and  for  impure  platinum  the 
value  of  6  increases  with  the  impurity.  Heycock  and  Neville 
made  a  study  of  the  effect  on  the  temperature  scale  obtained  by 
using  platinum  of  different  degrees  of  purity,  and  concluded, 
erroneously  it  now  appears,  due  to  an  incorrect  method  of  calcu- 
lation, that  thermometers  having  different  values  of  d  would  give 
the  same  temperature  scale  when  reduced  by  the  parabolic  for- 
mula of  Callendar. 

It  has  been  shown  since  that  impure  platinum  does  not  obey 
the  same  resistance-temperature  law  as  the  pure  metal,  and 
Waidner  and  Burgess  have  indicated  the  corrections  to  be 
applied  to  temperatures  obtained  by  the  Callendar  method, 
using  impure  platinum  to  reduce  to  the  usual  temperature 
scale.  They  find,  for  platinum  of  varying  degrees  of  purity 
as  indicated  by  the  values  of  5,  the  following  values  for  fixed 
points,  using  the  Callendar  equation  in  all  cases  for  computing 
the  temperatures: 


232  HIGH  TEMPERATURES 

FREEZING   POINTS   FOR   VALUES   OF   5   INDICATED   BELOW. 

5=  1.505  1-570  1.803 

Tin 231.90  231.82            

Zinc 4I9-37  419-32             

Antimony...  630.70  631.25             632.65 

Ag3-Cu2....  779-2  784-6 

Silver 960.9  966.2               975-3 

Copper 1083.0  1092.0  1106.0 

In  Table  VIII  of  the  Appendix  are  indicated  the  corrections 
to  be  applied  when  using  thermometers  of  impure  platinum. 
Wires  with  a  large  5  are  more  liable  to  change  with  use,  so  that, 
although  correct  results  may  be  obtained  with  them  if  properly 
reduced  and  checked  up  occasionally,  it  is  preferable  to  use  the 
purest  platinum. 

Changes  in  the  Constants.  —  If  platinum  thermometers  are  re- 
peatedly heated  to  temperatures  in  the  neighborhood  of  1000°  C., 
or  are  kept  for  very  considerable  periods  of  time  at  even  lower 
temperatures,  changes  in  the  value  of  the  constants  RQ,  Jf2i00, 
and  d  will  develop,  necessitating  frequent  recalibration  in  work 
of  high  accuracy.  Pyrometers  for  use  at  high  temperatures 
should  not  be  inclosed  in  inglazed  porcelain  even  if  the  glaze 
does  not  touch  the  metal,  as  deterioration  of  the  latter  will  other- 
wise ensue.  The  mica  supports  undergo  distortion  on  cooling 
from  high  temperatures,  increasing  in  size,  tending  to  stretch  the 
wire  and  increase  its  resistance.  For  this  reason  it  is  probably 
better  to  use  the  constants  determined  before  a  measurement 
at  high  temperature,  rather  than  those  determined  afterwards. 
Again,  if  the  wire  of  the  thermometer  has  not  been  well  an- 
nealed at  a  temperature  higher  than  it  is  to  be  used,  irregular 
changes  will  occur,  which  are  the  most  marked  for  the  first 
few  heatings. 

Waidner  and  Burgess  find  that  for  thermometers  of  pure 
platinum,  the  changes  in  their  constants  after  the  wires  have 
been  annealed  are  very  much  less  than  for  those  of  impure 
platinum;  thus,  as  shown  in  the  accompanying  table,  which  is 
typical,  Ro  changes  only  by  a  few  tenths  of  a  degree  for  pure 
platinum,  but  by  several  degrees  for  impure.  These  changes  are 


ELECTRICAL  RESISTANCE  PYROMETER  233 

CHANGES  IN  ZERO  OF  PLATINUM  THERMOMETERS. 


Thermometer  of  pure  platinum. 


Thermometer  of  impure  platinum. 


R0=3-  47971  at  start;         6  =  1.503. 
^0=3.48164  at  end;  diam.=  .15  mm. 

RO=  21  .3476  at  start;        5=1.570. 
Ro=2i  .0617  at  end;  diam.=  .  10  mm. 

Changes  in 
zero. 
°C. 

History  of  thermometer 
previously  annealed  at  1200°. 

Changes  in 
zero. 
°C. 

History  of  thermometer 
previously  annealed  at  1200°. 

-0.005  ' 
—     .001 

-f-  .007 

—    .002 
—   .050 

+  .013 

+  .138 

+  -144 

After  Zn  P.P.  3  times. 
After  Sb  P.P.  i  time. 
After  Sb  P.P.  2  times. 
After  2  hrs.  at  noo°+  C. 
After  Cu  P.P.  i  time. 
After  Cu  P.P.  2  times. 
After  Cu  P.P.  5  times. 
After  Ag  -Cu  P.P.  5  times. 

-0.18 
-    .29 
-2.27 

-3-94 
-4.66 

-5-99 
—  6.  20 
-6.46 

After  Zn  P.P.  10  times. 
After  Sb  P.P.  7  times. 
After  2  hrs.  at  noo°+  C. 
After  Cu  P.P.  i  time. 
After  Cu  P.P.  i  time.' 
After  Cu  P.P.  2  times. 
After  Ag  P.P.  2  times. 
After  Ag  -  Cu  P.P.  4  times. 

least  for  pure  platinum  wire  of  large  diameter  and  suspended 
free  from  strains.  For  impure  platinum  wire,  the  effect  of  high 
temperatures  is  to  decrease  RQ  and  to  increase  the  fundamental 
coefficient,  c\  that  is,  the  effect  is  as  if  the  wire  became  purer, 
possibly  because  of  the  evaporation  of  impurities,  for  example, 
indium.  If  the  platinum  is  pure,  the  slight  changes  indicate  a 
contamination  of  the  wire  and  the  effect  of  strains,  as  is  evidenced 
by  decrease  in  c  and  increase  in  RQ.  The  total  change  observed 
is  the  resultant  of  the  effects  of  strains,  of  annealing,  and  of 
contamination  and  purification. 

Use  of  Metals  other  than  Platinum.  —  Holborn  and  Wien 
found  that  with  palladium  the  absorption  of  hydrogen  at  low  tem- 
peratures, giving  the  hydride,  increases  the  resistance  by  60  per 
cent;  besides,  the  same  effect  of  alteration  as  with  platinum  is 
noticed  if  the  palladium  is  placed  in  hydrogen  in  the  presence  of 
silica.  Palladium  wound  on  mica  and  inclosed  in  porcelain  has 
been  shown  by  Waidner  and  Burgess  to  behave  in  a  very  similar 
manner  as  platinum  to  above  iooo°C.;  the  law  of  the  change 
of  resistance  of  palladium  with  temperature  is,  however,  very 
different  from  the  Callendar  equation,  and  is  an  equation  of 
the  fourth  degree  between  o°  and  1100°  C.  for  a  precision  better 


234  HIGH  TEMPERATURES 

than  0.5°  C.,  although  up  to  600°  the  Callendar  equation  is 
nearly  satisfied. 

No  very  definite  conclusion  is  to  be  drawn  from  the  work  of 
Holborn  and  Wien  with  iridium  and  rhodium,  except  that  these 
metals  assume  their  normal  resistance  only  after  being  heated 
several  times  to  a  high  temperature.  Iridium  evaporates  so 
much  more  readily  than  the  others  that  it  would  seem  the  least 
best  adapted  for  temperature  measurement  by  means  of  the 
metals  of  this  group,  and  platinum  is  evidently  to  be  preferred. 

Nickel  is  sometimes  used  in  resistance  thermometers,  but  it  is 
not -to  be  recommended  for  temperatures  above  300°  C.,  due  to 
the  change  in  the  resistance-temperature  relation  as  the  tran- 
sition temperature  of  nickel  is  approached  and  to  oxidation  at 
higher  temperatures.  Marvin  has  shown  that  for  pure  nickel 
the  equation  log  R  =  a  +  mt  holds  approximately  in  the  above 
limited  range  o°  to  300°  C. 

Conditions  of  Use.  —  The  electrical  resistance  pyrometer  of 
platinum  seems,  by  reason  of  the  great  precision  of  the  measure- 
ments which  it  allows,  to  be  especially  serviceable  for  laboratory 
investigations.  It  seems,  on  the  other  hand,  to  be  too  fragile  for 
many  of  the  industrial  applications  when  there  is  rough  handling, 
although  it  is  very  convenient  in  permanent  installations  when 
properly  protected,  and  when  it  is  desired  to  eliminate  completely 
the  often  troublesome  correction  necessary  for  the  temperature 
of  the  cold  junction  of  the  thermocouple. 

The  relation  between  the  platinum-thermometer  scale  and  the 
gas  scale  is  well  established  to  1100°  C.,  which  is  beyond  the 
limit  above  which  it  is  not  safe  to  use  the  platinum-resistance 
pyrometer  without  frequent  checking  of  its  calibration. 

The  resistance  pyrometer  is  the  best  instrument  for  differential 
work  and  for  detecting  small  temperature  changes  as  well  as 
for  controlling  a  constant  temperature.  It  is  also  particularly 
adapted  for  use  with  recording  instruments.  Great  care  has  to 
be  taken  that  the  platinum  does  not  become  contaminated. 

Industrial  Installations  and  Checking.  —  We  have  already 
called  attention  to  the  fragility  of  the  fire  end  of  a  resistance 


ELECTRICAL   RESISTANCE   PYROMETER 


235 


thermometer  and  the  necessity  for  protection  of  the  coil 
from  contact  with  furnace  gases. 
In  industrial  installations  it  is 
preferable  to  so  mount  the  py- 
rometer that  it  may  not  readily 
be  damaged  by  the  furnace  op- 
erations or  by  the  handling  of 
the  pyrometer  when  necessary  to 
withdraw  it.  This  can  usually 
be  done  by  a  suitable  arrange- 


ment of  the  pyrometer  within 
the  furnace  and  by  providing  a 
convenient  mechanism  for  with- 
drawing and  holding  the  py- 
rometer free  of  the  furnace.  A 
design  of  bracket  by  Leeds  and 
Northrup  is  shown  for  use  with 
a  small  oil-  or  gas-burning  fur- 
nace in  Fig.  83.  A  sliding  and 
slotted  collar  L  carries  the  py- 
rometer on  the  arm  Nj  the  whole  may  be  raised,  caught,  and 


Fig.  83.     Bracket  Mounting. 


Fig.  84.     Mountings  in  Oven. 

turned  on  the  pin  M,  permitting  the  removal  of  the  pyrom- 
eter  without    shock    and   providing    a    resting    place    without 


236  HIGH  TEMPERATURES 

handling  or  fear  of  breakage.  Proper  designs  for  mounting  py- 
rometers in  furnaces,  kilns  and  duct  pipes  are  shown  in  Figs. 
83,  84,  and  85.  If  the  pyrometer  tube  be  inserted  horizon- 
tally supported  only  at  one  end,  there  is  danger  of  bending 
and  breaking  even  when  the  outer  sheath  is  of  metal. 

The  resistance  pyrometer  and  its  electrical  circuit  may  be 
tested  in  place  and  the  calibration  verified  without  removal.  An 
industrial  installation  should  always  be  tested  for  proper  insula- 


Fig.  85.     Mounting  in  Duct. 

tion,  not  only  when  new  but  periodically,  or  when  irregular  be- 
havior occurs.  The  actual  operations  of  checking  out  the  insula- 
tion, lead,  and  contact  resistances  will  depend  upon  the  design  of 
the  instrument  and  the  voltage  for  which  it  is  intended.  It  is 
safe  to  say  that  the  resistance  between  any  wire  and  the  ground 
or  thermometer  case,  or  between  two  disconnected  wires  of  the 
system,  should  be  over  i  megohm  per  100  volts. 

Some  of  the  manufacturers  provide  outfits  for  the  automatic 
checking  of  thermometer  indicators  and  coils.  Thus,  Leeds  and 
Northrup  furnish  an  equipment  consisting  of  a  series  of  coils  cor- 


ELECTRICAL   RESISTANCE   PYROMETER  237 

responding  to  definite  temperatures  on  the  indicator,  and  another 
coil  equal  to  the  resistance  of  the  thermometer  at  room  tempera- 
ture. The  Cambridge  Company  also  furnish  "  ice  bobbins  "  by 
means  of  which  the  thermometer  resistance  may  be  checked  at 
o°  C.  The  use  of  the  resistance  pyrometer  industrially  is  also 
greatly  facilitated  by  the  practice  that  is  becoming  general  among 
makers  of  constructing  the  instruments  and  all  parts  so  that 
they  are  interchangeable.  This  is  particularly  necessary  for 
multiple  circuits  using  the  same  indicator  or  when  using  a  single 
automatic  recorder  in  connection  with  a  number  of  indicators. 
These  questions  are  considered  in  Chapter  X. 


CHAPTER  VI. 

THE  LAWS   OF  RADIATION. 

General  Principles.  —  The  temperature  of  bodies  may  be 
estimated  from  the  radiant  energy  they  send  out,  either  in  the 
form  of  visible  light  radiation  or  of  the  longer  infra-red  waves 
that  are  studied  by  their  thermal  effects.  For  the  estimation 
of  temperature  in  this  way  use  is  made  of  the  laws  of  radiation. 

Temperature  and  Intensity  of  Radiation.  —  When  we  consider 
the  enormous  increase  in  the  intensity  of  radiation  with  rise  in 
temperature,  this  method  appears  especially  well  adapted  to  the 
measurement  of  high  temperatures.  Thus,  for  example,  if  the 
intensity  of  the  red  light  (X  =0.65  /*)  emitted  by  a  body  at  1000°  C. 
is  called  i,  at  1500°  C.  the  intensity  will  be  over  130  times  as 
great,  and  at  2000°  C.  over  2100  times  as  great. 

The  rapid  increase  of  the  photometric  intensity  of  the  light  in 
comparison  with  that  of  the  temperatures  is  shown  by  the  follow- 
ing table,  from  Lummer  and  Kurlbaum,  for  light  emitted  by 
incandescent  platinum.  If  I\  and  72  are  the  intensities  of  the 
total  light  emitted  at  the  absolute  temperatures  T\  and  Tz  (not 
differing  many  degrees  from  one  another),  then  if  we  write  with 
Lummer  and  Pringsheim 


the  values  of  x  at  various  absolute  temperatures  (T°  C.  4-  273°) 
are  as  follows : 

T°  abs.  x. 

900°  30 

1000  25 

IIOO  21 

I2OO  IQ 

1400  18 

1600  15 

1900  14 

238 


THE  LAWS  OF  RADIATION  239 

From  this  table  it  will  at  once  be  seen  that  at  1000°  absolute 
(727°  C.)  the  intensity  of  the  light  increases  twenty-five  times  as 
rapidly  as  the  temperature;  at  1900°  absolute  (1627°  C.)  fourteen 
times  as  rapidly.  The  product  r#  =  25,000  as  shown  by  Rasch 
seems  to  express  the  relation  between  T  and  the  exponent  x. 

Emissive  Powers.  —  It  would  therefore  appear  that  a  system 
of  optical  pyrometry  based  on  the  intensity  of  the  light  emitted, 
by  incandescent  bodies  would  be  an  ideal  one,  inasmuch  as  a 
comparatively  rough  measurement  of  the  photometric  intensity 
would  measure  the  temperature  quite  accurately.  This,  however, 
is  only  partly  true ;  it  is  limited  somewhat  by  the  fact  that  different 
bodies,  although  at  the  same  temperature,  emit  vastly  different 
amounts  of  light.  Thus  the  intensity  of  the  radiation  from  in- 
candescent iron  or  carbon  at  1000°  C.,  for  example,  is  many  times 
greater  than  that  emitted  by  such  substances  as  magnesia, 
polished  platinum,  etc.,  at  the  same  temperature.  Consequently, 
if  any  conclusions  were  drawn  as  to  the  temperatures  of  these 
bodies  from  the  light  that  they  emit,  it  might  lead  to  large  errors. 
Thus  at  1500°  C.  this  difference  in  the  intensity  of  the  light 
emitted  by  carbon  and  by  polished  platinum  would  lead  to  a 
difference  in  the  estimated  temperature  of  these  bodies  of  about 
1 00°  C.,  and  less  at  lower  temperatures. 

The  "Black  Body"  —  Kirchhoff  in  one  of  the  most  important 
contributions  to  the  theory  of  radiation  was  led  to  the  important 
conception  of  what  he  termed  a  "black  body,"  which  he  defined 
as  one  which  would  absorb  all  radiations  falling  on  it,  and  would 
neither  reflect  nor  transmit  any.  He  further  pointed  out  clearly 
the  important  fact  that  the  radiation  from  such  a  black  body  was 
a  function  of  the  temperature  alone,  and  was  identical  with  the 
radiation  inside  an  inclosure  all  parts  of  which  have  the  same 
temperature.  Various  expressions  are  in  use  for  the  "  black 
body,"  such  as  "integral  radiator,"  "full  radiator,"  etc. 

Experimental  Realization.  —  The  first  experimental  realization 
of  a  black  body  as  a  practical  laboratory  apparatus  was  made 
by  Lummer  and  Wien,  by  heating  the  walls  of  a  hollow  opaque 
inclosure  as  uniformly  as  possible  and  observing  the  radiation 


240  HIGH  TEMPERATURES 

coming  from  the  inside  through  a  very  small  opening  in  the  walls 
of  the  inclosure.  No  substance  is  known,  however,  whose  surface 
radiation  is  exactly  that  of  a  black  body.  The  radiations  from 
such  substances  as  carbon  and  iron  oxide  approximate  fairly  near 
to  black-body  radiation,  while  such  bodies  as  polished  platinum 
and  magnesia,  etc.,  depart  very  far  from  it.  Black-body  radia- 
tions, corresponding  to  temperatures  from  that  of  liquid  air  or 
lower,  up  to  2500°  C.  or  higher  (if  suitable  materials  are  chosen), 
are  now  available  in  the  laboratory.  For  temperatures  up  to 
600°  or  thereabouts,  this  is  realized  by  immersing  a  metallic  or 
other  vessel  in  a  constant-temperature  bath  (liquid,  gas,  vapor, 
or  fused  salt)  and  observing  the  radiation  from  the  interior 
through  a  small  opening  in  the  walls.  At  higher  temperatures 
it  is  very  difficult  to  heat  the  walls  of  the  inclosure  uniformly, 
especially  with  gas  flames.  Lummer  and  Kurlbaum  have  very 
satisfactorily  overcome  this  difficulty  in  their  electrically  heated 
black  body,  which  is  shown  in  section  in  Fig.  86. 

The  central  porcelain  tube  is  wound  over  with  thin  platinum 
foil  through  which  an  electric  current  is  sent  which  can  be  ad- 
justed to  maintain  any  desired  temperature  up  to  1500°  C.  This 
tube  is  provided  with  a  number  of  diaphragms  to  minimize  the 
disturbing  effects  of  air  currents.  To  protect  this  inner  tube 
from  external  influences  and  to  diminish  unnecessary  heat  losses, 
it  is  surrounded  by  several  porcelain  tubes  and  air  spaces,  as 
shown  in  the  figure.  The  radiation  from  the  uniformly  heated 
region  near  the  center,  and  which  passes  out  through  the  end  of 
the  tube  at  0,  is  a  very  close  approximation  of  the  ideal  black- 
body  radiation  of  Kirchhoff.  The  temperature  of  this  central 
region  is  measured  by  means  of  one  or  more  carefully  calibrated 
thermocouples.  By  adding  supplementary  heating  coils  at  the 
ends  the  temperature  distribution  may  be  improved.  Waidner 
and  Burgess  were  able  to  obtain  a  constancy  of  i°  C.  throughout 
the  greater  part  of  the  length  of  such  an  apparatus.  The  cali- 
bration of  optical  and  radiation  pyrometers  is  carried  out  by 
means  of  such  a  black  body.  For  higher  temperatures  special 
furnaces  are  used,  which  we  shall  describe  later. 


THE   LAWS  OF   RADIATION 


241 


3 


a 


242  HIGH  TEMPERATURES 

As  has  already  been  stated,  if  magnesia,  porcelain,  platinum, 
iron,  etc.,  are  heated  to  the  same  temperature,  they  will  emit 
vastly  different  amounts  of  light.  If,  however,  these  bodies* 
are  heated  inside  a  black  body,  they  will  all  emit  the  same  radia- 
tion, and  on  looking  into  the  small  opening  all  details  of.  their 
contour  will  be  lost,  the  whole  region  being  of  uniform  brightness. 
Thus,  in  the  black  body  described  above,  before  the  heating  has 
become  uniform,  the  platinum  wires  of  the  thermocouple  can  be 
seen  as  dark  lines  against  the  brighter  background,  but  when  the 
heating  current  has  been  maintained  constant  for  some  time,  so 
that  the  heating  has  become  uniform  in  the  inner  central  chamber, 
the  wires  of  the  couple  almost  completely  disappear,  notwith- 
standing that,  of  all  substances,  platinum  and  the  black  oxide  of 
the  radiating  walls  differ  most  widely  in  their  radiating  powers 
(emissivities). 

Realization  in  Practice.  —  Fortunately,  in  pyrometric  practice 
it  is  often  easy  to  realize  very  nearly  the  conditions  of  a  black 
or  totally  absorbing  body.  Thus  the  interior  of  most  furnaces, 
kilns,  and  ovens  approximates  this  condition,  or  the  bottom  of  a 
closed  tube  of  any  material  thrust  into  any  space  heated  to  in- 
candescence. Again,  iron  and  coal  observed  in  the  open  are  not 
far  removed  in  their  optical  properties  from  the  black  body. 

Black-body  Temperature.  —  The  term  "  black-body  tempera- 
ture "  has  come  into  quite  extensive  use  and  is  of  great  conven- 
ience in  the  discussion  of  pyrometric  problems.  The  tempera- 
tures indicated  by  a  radiation  pyrometer  that  has  been  calibrated 
against  a  black  body  are  known  as  black-body  temperatures. 
Thus,  were  a  piece  of  iron  and  a  piece  of  porcelain  both  at  1200°, 
the  optical  pyrometer,  which  used  the  red  light  emitted  by  these 
bodies,  would  give,  as  the  temperature  of  these  bodies,  1140° 
and  1 1 00°  respectively.  This  means  that  iron  and  porcelain  at 
1200°  emit  red  light  of  the  same  intensity  as  is  emitted  by  a 
black  body  at  1140°  and  noo°C.  respectively.  The  "  black- 

*  It  is  here  assumed  that  the  radiation  is  purely  thermal  and  that  no 
part  is  due  to  luminescence,  as  the  laws  of  radiation  are  only  directly  appli- 
cable where  such  is  the  case. 


THE   LAWS  OF  RADIATION  243 

body  temperature  "  of  these  materials  for  green  light  might  differ 
quite  appreciably  from  that  for  red  light.  It  is  at  once  evident 
that  if  the  "  black-body  temperatures  "  of  different  bodies,  e.g., 
carbon  and  platinum,  are  equal,  their  actual  temperatures  may 
differ  considerably  (180°  C.,  or  so,  at  1500°  C.).  This  violates 
our  ordinary  conception  of  equal  temperatures,  which  is  based 
on  thermal  equilibrium  between  the  bodies  if  brought  into 
contact.  The  term  "  equivalent  temperature,"  suggested  by 
Guillaume,  is  also  used  for  "  black-body  temperature."  Waidner 
and  Burgess  have  suggested  the  notation:  1500°  K&,  meaning 
1500°  absolute  centigrade  as  viewed  with  light  of  wave  length 
0.65/4. 

The  temperature  of  any  body,  therefore,  as  measured  by  an 
optical  or  by  a  radiation  pyrometer,  will  always  be  lower  than  its 
true  temperature  by  an  amount  depending  on  the  departure  of 
its  radiation  from  that  of  a  black  body.  There  is  another  source 
of  error,  however,  that  may  act  in  the  direction  of  making  the 
pyrometer  read  too  high,  due  to  light  reflected  by  the  body  whose 
temperature  is  being  measured.  This  source  of  error  may  very 
often  be  eliminated,  where  the  accessibility  of  the  work  permits, 
by  running  a  tube  down  to  the  incandescent  surface,  which 
will  cut  off  stray  radiation  from  the  surrounding  flames.  The 
magnitude  of  the  error  that  may  arise  from  light  reflected  from 
surrounding  hotter  objects  may  be  quite  considerable  (several 
hundred  degrees),  depending  on  the  temperature,  area,  and  posi- 
tion of  the  surrounding  hot  objects  and  the  reflecting  power  of 
the  surface  whose  temperature  is  under  observation. 

Kirchhoff 's  Law.  —  If  we  consider  an  opaque  object  and  let 
radiation  fall  upon  it,  the  relation  between  the  proportions  re- 
flected (r)  and  absorbed  (a)  is: 

r  +  a  =  i. 

For  such  objects,  therefore,  eliminating  the  effects  of  polarized 
light  and  angle  of  incidence  and  assuming  we  are  dealing  with 
matt  surfaces  and  thermal  radiation  only,  the  determination 
of  either  the  absorbing  or  reflecting  power  gives  also  the  other. 


244  HIGH  TEMPERATURES 

The  quantity  a  depends  on  the  nature  of  the  substance  and  is  a 
function  of  the  wave  length  and  temperature  only,  or 

a  =f(\,T). 

For  a  radiating  body  the  emissive  power  e  is  a  similar  function 
of  X  and  T. 

By  definition,  a  black  body  absorbs  all  the  radiation  incident 
upon  it,  therefore  in  this  case  a  =  i  and  r  =  o  for  all  values  of 
X  and  T.  The  emissive  power  €  of  a  radiating  black  body  is 
evidently  fundamental  to  the  theory  of  radiation,  and  the  func- 
tion e  =  F(\,T)  forms  the  basis  of  several  of  the  radiation  laws. 

KirchhofFs  law  as  applied  to  monochromatic  radiation  may 
be  stated  in  the  following  way:  e  =  ae,  or  more  completely: 


a  an 

The  ratio  of  the  emission  to  the  absorption  is  for  all  bodies  the 
same  function  of  wave  length  and  temperature,  and  is  equal  to  the 
emission  of  a  black  body. 

There  are  a  number  of  corollaries  to  KirchhofFs  law,  some  of 
which  we  may  emphasize  as  being  of  interest  in  temperature 
measurements,  bearing  in  mind  that  we  are  here  dealing  only 
with  radiation  due  to  thermal  causes. 

The  emissive  power  of  a  black  body  is  greater  than  that  of  any 
other  body  at  the  same  temperature.  Every  body  absorbs  the 
same  rays  that  it  emits  at  a  given  temperature.  It  may  also 
absorb  other  rays,  but  they  will  be  among  those  that  a  black 
body  does  not  emit  at  the  given  temperature. 

In  general,  the  ratio  e  :  a,  which  is  the  same  for  all  bodies  for 
given  values  of  X  and  T,  does  not  depend  on  the  degree  or  kind 
of  polarization  of  the  radiation. 

The  energy  curves  e  =/(X)  for  each  value  of  T  lie  wholly 
within  the  corresponding  black-body  curves  e  =  /(X). 

In  the  case  of  composite  radiations,  that  is,  of  spectral  bands 
having  for  limiting  case  the  whole  spectrum,  KirchhofFs  law  ap- 
plies only  under  special  conditions.  Thus  KirchhofFs  law  holds 
for  any  composite  radiations,  taken  between  the  limits  Xi  and 


THE  LAWS  OF  RADIATION  245 

^2,  if  the  total  absorption  is  referred  to  the  radiation  from  a 
black  body  at  the  same  temperature  as  the  bodies  to  be  compared. 

Again,  Kirchhoff's  law  holds  for  composite  radiations,  when 
the  two  given  bodies  are  at  the  same  temperature  and  when  each 
of  them  serves  as  source  to  the  radiation  which  measures  the 
total  absorption  of  the  other. 

A  corollary  of  considerable  practical  importance  is  the  follow- 
ing: If  two  surfaces  of  any  substances  whatever  at  the  same 
temperature  radiate  only  on  each  other,  the  radiation  from  each 
is  equivalent  to  the  emission  of  a  black  body;  from  which  it 
follows  that,  within  an  inclosed  space  at  constant  temperature, 
all  bodies  emit  radiation  identical  to  that  of  a  black  body.  And 
finally,  the  radiation  from  a  small  opening  in  such  an  inclosure 
at  constant  temperature  is  black-body  radiation,  and  depends 
only  on  the  temperature. 

It  is  worthy  of  remark  that  in  the  measurement  of  radiation 
from  such  a  black  body  the  receiver  should  also  be  a  black  body, 
or  at  least  its  coefficient  of  absorption  should  be  known  for  the 
kind  of  radiation  to  be  studied,  if  the  radiation  laws  as  applied 
to  a  black  body  are  assumed  to  hold,  as  is  often  the  case. 

Stefan's  Law.  —  Naturally  the  first  numerical  relation  sought 
between  intensity  of  radiation  and  temperature  was  one  for  the 
total  energy  of  radiation  sent  out  by  a  body,  as  it  required  less 
delicate  instruments  for  measurement  than  the  study  of  the 
spectral  distribution  of  energy.  Numerous  attempts  to  express 
such  a  relation  were  made  by  Newton,  Dulong  and  Petit,  Rosetti, 
and  others.  These  attempts,  however,  merely  resulted  in  empiri- 
cal expressions  that  held  only  through  narrow  ranges  of  tempera- 
ture. The  first  important  step  was  made  by  Stefan,  who  ex- 
amined some  of  the  experimental  data  of  Tyndall  on  the  radiation 
of  incandescent  platinum  wire  in  the  interval  525°  C.  to  1200°  C., 
and  was  led  to  the  conclusion  that  the  energy  radiated  was. 
proportional  to  the  fourth  power  of  the  absolute  temperatures. 
This  relation  seemed  to  be  further  supported  by  the  best  experi- 
mental data  of  other  observers,  at  least  to  within  the  limit  of 
accuracy  of  their  observations,  being  strictly  true,  however,  only 


246 


HIGH  TEMPERATURES 


for  the  energy  of  total  radiation  from  a  black  body.  This  rela- 
tion received  independent  confirmation  from  Boltzmann,  who 
deduced  it  from  thermodynamic  reasoning.  The  conditions  im- 
posed by  Boltzmann  in  his  discussion  on  the  nature  of  the  radi- 
ation were  such  as  are  fulfilled  by  the  radiation  from  a  black 
body.  This  relation,  which  has  now  come  to  be  generally  known 
as  the  Stefan-Boltzmann  radiation  law,  may  then  be  stated  as 
follows : 

The  total  energy  radiated  by  a  black  body  is  proportional  to  the 
fourth  power  of  the  absolute  temperature,  or 


(B) 


when  E  is  the  total  energy  radiated  by  the  body  at  absolute 
temperature  T°  to  the  body  at  absolute  temperature  T°,  and  a- 
is  a  constant  depending  on  the  units  used.  Usually  TQ  is  small 
compared  with  T,  so  that  practically  we  may  write 

(Br)  E  =  oT4. 

This  law  has  received  abundant  experimental  support  from  the 
researches  of  Lummer,  Kurlbaum,  Pringsheim,  Paschen,  and 
others,  throughout  the  widest  range  within  which  temperature 
measurements  can  be  made. 

An  illustration  of  the  experimental  evidence  in  support  of  this 
law  is  given  in  the  table  taken  from  the  experiments  of  Lummer 
and  Kurlbaum. 


E 

r*  -  r0« 

r 

n 

Black  body. 

Polished 
platinum. 

Iron  oxide. 

372.8 

290.5 

108.9 

492 

290 

109.0 

2.28 

33-1 

654 

290 

108.4 

6.56 

33-1 

795 

290 

109.9 

8.14 

36.6 

1108 

290 

109.0 

12.  18 

46.9 

1481 

290 

110.7 

16.69 

65.3 

1761 

290 

19.64 

THE   LAWS   OF   RADIATION  247 

It  will  also  be  seen  from  this  table  that  while  the  intensity  of 
the  total  radiation  of  iron  oxide  is  4  or  5  times  that  of  polished 
platinum,  it  is  still  considerably  less  than  that  emitted  by  a  black 
body.  The  total  radiation  from  objects  other  than  a  black  body 
increases  more  rapidly  than  the  fourth  power  of  the  absolute 
temperature,  so  that  as  the  temperature  is  raised  the  radiation  of 
all  bodies  appears  to  approach  that  of  the  black  body.  Whether 
or  not  there  is  a  maximum  limiting  value  of  radiation  due  to 
purely  thermal  causes,  is  still  an  unsettled  question,  however. 

The  numerical  value  of  the  constant  a  for  a  black  body  is  of 
interest  in  absolute  measurements  and  in  checking  the  constants 
of  radiation  instruments.  For  the  radiation  per  degree  C.  from 
i  cm.2,  expressed  in  gram-calories  per  second,  the  values  found 
for  a-  range  from  less  than  i.o-io~12  by  Bottomly  and  King 
to  1.52  •  io~12  by  Fery.  The  following  observers,  however,  have 
obtained  results  agreeing  more  closely: 

Kurlbaum,  1.277  •  I0~12 

Valentiner,  1.286 

Bauer  and  Moulin,    1.275 

,,  _19  19  watts 

Mean  =  1.279  •  10       °  5-34  * I0  2  ' 

cm. 

The  last  two  series  of  measurements  were  carried  out  over  very 
extended  temperature  intervals  —  in  the  case  of  Valentiner  to 
nearly  1600°  C.  His  results  correspond  to  the  temperature  scale, 
established  by  Holborn  and  Valentiner.  It  should  be  noted, 
however,  that  if  compensating  errors  are  present  in  the  energy 
measurements  or  in  the  non-blackness  of  the  radiator  or  receiver, 
it  is  possible  to  obtain  a  correct  value  of  <r  from  an  incorrect 
temperature  scale,  as  is  evident  from  the  logarithmic  form  of  the 
Stefan-Boltzmann  equation:  log  E  =  log  a  +  n  log  T. 

Laws  of  Energy  Distribution.  —  Among  the  first  facts  to  be 
noticed  about  the  nature  of  the  radiations  sent  out  by  bodies 
were,  that  at  low  temperatures  these  radiations  consisted  of 
waves  too  long  to  affect  the  human  eye.  As  the  temperature 
was  raised,  shorter  and  shorter  waves  were  added,  which  could 


248  HIGH   TEMPERATURES 

finally  be  detected  by  the  eye;  the  first  of  the  visible  radiations 
producing  the  sensation  called  red,  then  orange,  etc.,  until  the 
violet  waves  were  reached,  which  were  the  shortest  waves  that 
the  eye  could  detect. 

Soon  after  Langley  brought  out  the  bolometer,  which  was  so 
admirably  adapted  to  the  measurement  of  the  minute  energy 
of  radiations,  a  great  mass  of  valuable  experimental  data  was 
obtained,  bearing  on  the  spectral  distribution  of  the  energy  of 
the  radiation  emitted  by  various  bodies.  Among  the  most  im- 
portant of  these  contributions  must  be  mentioned  the  researches 
of  Paschen,  who  examined  the  distribution  of  energy  in  the  emis- 
sion and  absorption  spectra  of  various  substances.  Among  the 
experimental  facts  established  by  these  researches  were,  that  by 
far  the  largest  portion  of  the  energy  in  the  spectrum  was  found 
in  the  infra-red  region,  that  the  position  of  the  wave  length  hav- 
ing the  maximum  energy  depended  on  the  temperature  of  the 
body,  and  that,  as  the  temperature  was  raised,  the  energy  of  all 
the  waves  emitted  increased,  but  the  shorter  waves  more  rapidly 
than  the  longer,  so  that  the  position  (wave  length)  of  maximum 
energy  in  the  spectrum  shifted  toward  shorter  wave  lengths. 
These  facts  are  well  illustrated  by  the  curves  shown  in  Fig.  85, 
taken  from  a  paper  by  Lummer  and  Pringsheim,  in  which  the 
ordinates  are  proportional  to  the  intensity  of  radiation  emitted 
by  a  black  body,  and  the  abscissas  are  wave  lengths  (in  thou- 
sandths of  a  millimeter).  Such  curves,  as  are  here  shown,  where 
the  temperature  is  constant  and  the  energy  is  measured  corre- 
sponding to  radiations  of  different  wave  lengths  emitted  by  a 
body,  are  called  energy  curves,  i.e.,  the  relation  determined  is 
/  =/(X)  for  T  —  constant,  where  7  =  energy  corresponding  to 
wave  length  X,  strictly  the  energy  comprised  in  the  region  of  the 
spectrum  between  X  and  X  +  d\,  and  T  is  the  absolute  tempera- 
ture of  the  radiating  source.  It  is  also  interesting  to  study  the 
change  in  the  intensity  of  some  particular  wave  length  as  the 
temperature  of  the  radiating  source  is  changed,  i.e.,  to  find 
/  =  F  (T)  for  X  =  constant.  This  can  of  course  be  done  by 
exposing  the  bolometer  strip  in  a  fixed  part  of  the  spectrum  and 


THE  LAWS  OF   RADIATION 


249 


observing  the  galvanometer  deflections  as  the  temperature  is 
changed.  The  curves  obtained  in  this  way  for  /  =  F  (T)  are 
called  isochromatic  curves. 

Wien's  Laws.  —  Wien  was  led  from  theoretical  considerations 
to  state  that  "  when  the  temperature  increases,  the  wave  length 


1650 


ENERGY  CURVE'S 

FOR 
BLACK  BODY, 


j 
2 


Fig.  87.     Energy  Curves. 


of  every  monochromatic  radiation  diminishes  in  such  a  way  that 
the  product  of  the  temperature  and  the  wave  length  is  a  constant," 

\T  =  XoZV 

Hence  for  the  wave  length  of  the  maximum  energy,  Xw,  we  have 

=  const.    ..    ...  .    .     .     .     .     (I) 


250  HIGH   TEMPERATURES 

This  is  known  as  the  "  Wien  displacement  law  "  and  is  simply  a 
mathematical  statement  of  the  fact  that  as  the  temperature  of 
the  radiating  source  is  changed  the  wave  length  having  maxi- 
mum energy  in  the  spectrum  will  be  changed  in  such  a  way  that 
the  product  of  this  wave  length  and  the  corresponding  absolute 
temperature  of  the  source,  T,  is  equal  to  a  constant.  Wien  then 
combined  the  above  relation  with  the  Stefan-Boltzmann  law  and 
was  led  to  the  relation  that 

£,...       .       .       (II) 


in  which  7max.  indicates  the  energy  corresponding  to  the  wave 
length  of  the  maximum  energy  and  T  is  the  absolute  temperature 
of  the  radiating  source  (black  body).  Both  of  these  generaliza- 
tions of  Wien  for  the  radiations  emitted  by  a  black  body  have 
received  the  most  convincing  experimental  verification  through- 
out the  widest  ranges  of  measurable  temperatures  that  are  at 
present  available  to  the  experimentalist. 

As  an  illustration  of  the  experimental  evidence  in  support  of 
these  two  laws  of  radiation,  the  following  table  has  been  added, 
taken  from  a  paper  by  Lummer  and  Pringsheim  on  the  radiation 
from  a  black  body: 


K       T     T   &  Absolute 

B=ImT-s          temperature. 


/     I 

\  j 


4.53     2.026  2814  2190.  io~17    621.2°  621.3  +0.1 

4.08     4.28  2950  2166        723.0  721.5  —1-5 

3.28  13.66  2980  2208        9°8-5  910.1  +1.6 

2.96  21.50  2956  2166        998.5  996.5  —  2.0 

2.71  34-Q  2966  2164  1094.5  1092.3  —2.2 

2.35  68.8  2959  2176  1259.0  1257.5  -i-S 

2.04  145.0  2979  2184  1460.4  1460.0  —0.4 

1.78  270.8  2928  2246  1646.0  1653.5  +7-5 


Mean 2940        2188.  io~17 

As  will  be  seen,  these  results  of  experiment  are  in  most  satis- 
factory agreement  with  these  laws,  when  one  considers  the  experi- 
mental difficulties  that  are  involved  in  the  measurements.  In 
the  value  for  B  the  temperature  enters  to  the  fifth  power,  so  that 
a  small  error  in  the  temperature  produces  a  very  marked  effect 
•on  the  value  of  B.  Paschen  later  obtained  \mT  =  2920. 


THE   LAWS   OF   RADIATION  251 

Wien  also  published  the  result  of  a  further  theoretical  investi- 
gation on  the  spectral  distribution  of  energy  in  the  radiation  of  a 
black  body,  in  which  he  was  led  to  the  conclusion  that  the  energy 
I  corresponding  to  any  wave  length  was  represented  by 


where  /  is  the  energy  corresponding  to  wave  length  X,  T  is  the 
absolute  temperature  of  the  radiating  black  body,  e  is  the  base 
of  the  natural  system  of  logarithms,  and  c\  and  c2  are  constants. 
We  have  further  the  relation:  c2  =  5  \mT,  to  which  we  may  assign 
the  value  c2  =  14,500. 

The  subsequent  experimental  work  of  Beckman,  Rubens,  and 
others  has  shown  that  Wien's  distribution  law  does  not  hold  for 
long  wave  lengths,  although  it  amply  suffices  throughout  the 
whole  visible  spectrum,  and  may  be  applied  in  all  cases  where 
xr  <  3000. 

Planck  has  deduced  an  expression  analogous  to  Wien's  which 
applies  with  exactness  for  all  wave  lengths  and  temperatures. 
His  law,  which  reduces  to  Wien's  for  small  values  of  X,  may  be 
written 


7=c,X-Vr-iy    , (IV) 

in  which  c2  =  4.965  XmT,  a  more  exact  solution  than  that  given 
by  Wien's  law  III. 

Other  radiation  laws  have  also  been  suggested,  but  Planck's 
seems  to  best  satisfy  both  experiment  and  theory. 

For  the  radiation  from  all  substances  that  have  been  examined 
experimentally,  it  has  been  found  generally  that  the  "displace- 
ment law," 

Xmir  =  const.  =  Ai, (la) 

still  holds  true,  although  the  radiation  may  depart  far  from  that 
for  a  black  body.  In  this  case,  however,  the  value  of  the  con- 
stant is  different  from  that  for  a  black  body.  Thus  for  polished 
platinum  Lummer  and  Pringsheim  found  AI  =  2626. 


252 


HIGH  TEMPERATURES 


For  the  radiation  from  other  than  a  black  body  the  law  of 
maximum  energy  applies  only  in  the  modified  form 


ImT~a  =  const.  = 


(Ila) 


where  a  cannot  be  less  than  5  and  is  not  probably  ever  greater 
than  6,  the  value  found  by  Lummer  and  Pringsheim  for  polished 
platinum.  The  general  form  of  Wien's  law  (III)  takes  the  form 


/_  /-\— a^     XT1.  (TlT\ 

=  Ci\       €  , (LLL  ) 

where  6  >  a  >  5. 

The  corresponding  form  assumed  by  Stefan's  law  for  non-black 
bodies  is 

E  =  c/r"-1,     . (BO 

where  a  is  the  same  as  in  the  preceding  equation. 

Lummer  and  Pringsheim  found  the  following  limits  of  tempera- 
ture as  given  by  the  Wien  relation  (la) : 


\m 

Tmai. 

Train 

Electric  arc 

O   7u 

4200  abs. 

37<co  abs. 

Nernst  lamp 

2 

24  so 

2  2OO 

Auer  burner 

2 

2A.ZO 

22OO 

Incandescent  lamp 

4 

2  TOO 

1871; 

Candle 

e 

IO6O 

17^0 

Argand  burner  .        .        ...        

.  "^ 

IOOO 

1  700 

Lummer  and  Pringsheim  also  heated  a  carbon  tube  electrically 
to  about  2000°  C.  and  observed  the  temperature  inside  simulta- 
neously with  instruments  making  use  of  the  several  radiation 
laws: 

Method.  T  absolute. 

(  2310 
Photometric  ........................  <  2320 

'  2330 


Total  radiation  .....................  <  2345 

'  2325 

Energy  maximum  ....  ...............  <    ^ 

This  complete  concordance  at  such  a  high  temperature  between 
the  different  radiation  methods  gives  further  confidence  in  the 


THE  LAWS  OF  RADIATION  253 

legitimacy  of  their  indefinite  extrapolation  for  nonluminescent 
bodies.  Waidner  and  Burgess  have  also  found  that  this  accord 
probably  exists  at  the  temperature  of  the  electric  arc,  3600°  C. 

In  spite  of  the  excellent  agreement  among  the  above-mentioned 
experiments  in  confirming  the  validity  for  temperature  meas- 
urements of  the  several  radiation  laws,  there  is,  nevertheless, 
too  great  an  outstanding  uncertainty  in  the  numerical  values  of 
the  characteristic  constants  of  the  equations  representing  these 
laws,  such,  for  instance,  as  the  a  in  Stefan's  law  (B),  page  246,. 
and  the  c2  in  the  Wien  equation  (III),  page  251,  for  black-body 
radiation,  as  well  as  in  the  corresponding  quantities,  and  par- 
ticularly the  value  of  the  exponent  a  (equation  (II),  etc.),  for 
other  substances. 

We  have  seen  (page  80)  that  there  is  a  discrepancy  of  con- 
siderable magnitude,  of  at  least  25°  C.,  at  the  melting  point  of 
palladium,  on  the  scale  of  the  nitrogen-gas  thermometer  as  given 
by  Holborn  and  Valentiner  and  by  Day  and  Sosman.  The 
measurements  of  the  former,  made  simultaneously  with  the  gas- 
thermometer  determinations,  lead  to  a  value  of  c<i  =  14,200  in 
Wien's  equation  (III),  and  in  the  hands  of  Valentiner  to  a  con- 
stant value  of  5.34  watts  for  a  in  Stefan's  equation  (B).  The 
gas  scale  of  Day  and  Sosman,  however,  corresponds  more  nearly 
to  a  value  of  c2  =  14,500  as  deduced  from  the  optical  measure- 
ments of  Nernst  and  Wartenberg  and  of  Waidner  and  Burgess. 
It  is  very  important  that  this  outstanding  discrepancy  be  settled, 
and  further  work  is  in  progress  in  several  laboratories  on  these 
and  allied  subjects. 

Warburg  and  Leithauser,  in  a  preliminary  announcement  of 
new  determinations  of  c%  made  at  the  Reichsanstalt,  find  values 
ranging  between  14,200  and  14,600,  depending  on  the  experi- 
mental conditions.  Their  best  value  so  far  announced  is  14,570. 

Coblentz  at  the  Bureau  of  Standards  has  also  published  pre- 
liminary results  on  cz  approximating  14,600. 

Applications  to  Pyrometry.  —  It  is  evident  that  theoretically 
any  of  these  laws  and  their  various  consequences  might  be  used 
as  a  basis  of  pyrometry ,  but  practically  it  is  not  convenient . 


254  HIGH  TEMPERATURES 

to  make  use  of  all  of  them.  The  displacement  law  (\mT=A) 
and  the  maximum-energy  law  (ImT~5  =  B)  of  Wien  are  well- 
established  relations,  but  in  practice  it  is  exceedingly  difficult  to 
construct  instruments  of  sufficient  sensibility  to  give  any  consid- 
erable precision,  and  any  industrial  pyrometer  using  these  prin- 
ciples is  out  of  the  question  at  the  present  time.  The  reason 
of  the  lack  of  sensibility  with  the  relation  X™  T  —  A  is  due  to  the 
fact  that  the  exact  position  of  the  wave  length  possessing  the 
maximum  of  energy  is  very  difficult  to  locate,  especially  at  rela- 
tively low  temperatures  (see  Fig.  87).  The  value  of  the  maxi- 
mum energy  could  perhaps  be  measured  more  readily,  but  as 
this  quantity  varies  as  the  fifth  power  of  the  temperature,  there 
would  hardly  be  any  preference  for  this  over  the  former  method. 
There  have  been,  however,  several  most  convenient,  simple, 
and  very  accurate  instruments  devised  which  are  based  either  on 
the  use  of  Stefan's  law,  E  =  a  (T4—  TV),  or  Wien's  distribution 

__c1 

law,  /  =  CiX~5e  xr,  either  directly  or  indirectly,  and  in  the  two 
following  chapters  we  shall  treat  of  these  at  some  length. 

The  last  equation  is  conveniently  put  into  the  logarithmic 
form  for  computation: 

l 


or,  when  comparing  two  intensities,  as  in  temperature  measure- 
ments, the  determination  of  an  absorption  or  reflection  coefficient, 
or  when  comparing  one  type  of  radiation  with  another,  we  have: 


By  expressing  numerical  results  graphically,  it  is  evident  that 
log  I  vs  —  is  a  straight  line,  which  greatly  simplifies  the  reduction 

of  observations.  It  will  be  seen  also  that  the  constant  c%  must 
be  known  exactly  if  (Ilia)  is  to  be  used  as  a  basis  of  correct  tem- 
perature measurement.  Unfortunately  there  is  still  a  consider- 
able lack  of  certainty  in  our  knowledge  of  this  constant,  as  shown 


THE  LAWS  OF  RADIATION  255 

above,  although  its  value  has  been  shown  by  several  observers 
to  remain  sufficiently  constant  throughout  the  visible  spectrum. 
We  shall  use  the  value  c<t  =  14,500  in  what  follows. 

It  has  been  shown  by  Henning,  v.  Wartenberg,  and  others  that 
the  absorption  coefficient  a,  for  the  various  wave  lengths  of  the 
visible  spectrum,  does  not  change  appreciably  with  temperature, 
at  least  for  several  of  the  metals;  and  this  fact  may  be  taken 
advantage  of  in  temperature  measurements  when  sighting  on  an 
object  for  which  we  know  the  value  of  the  absorption  coefficient 
#,  the  reflection  coefficient  r  =  i  —  a,  or  the  emissivity  e  =  a 
when  that  of  a  black  body  is  taken  as  unity.  If  S\  is  the  black- 
body  temperature  absolute  (=  s  +  273),  that  is,  the  apparent 
temperature  of  the  substance  as  given  by  an  optical  pyrometer, 
using  light  of  wave  length  X,  and  T  (  =  t  +  273)  the  corresponding 
true  temperature  of  the  substance,  then  Wien's  equation  gives: 


r    sx    c2  log 

This  form  of  Wien's  equation  is  of  the  greatest  importance  in 
the  practical  application  of  optical  pyrometry  to  the  estimation 
of  the  temperature  of  incandescent  metals  and  other  substances 
for  which  the  absorption  coefficients  are  known. 

It  is  also  possible  to  estimate  a  temperature  by  Wien's  law 
from  the  measurement  of  the  light  intensity  at  two  wave  lengths. 
The  general  expression  is: 

i     1  1        i     A2  ,  £2  log  e  1  1       i  \  ,  ,      a\  /,  ,x 

log-1  =  5log-  +  —  -f-  -  --)  +  logJ      .     .     (V) 

lz  AI  1          \A2         A]./  #2 

where  a\  and  02  are  the  absorption  coefficients  for  the  wave  lengths 
\i  and  \2,  and  where  I\  and  /2  are  the  corresponding  intensities 
corrected  for  the  sensibility  of  the  eye.  In  the  case  of  a  black 
body,  the  last  term  is  zero.  The  eye-sensibility  correction  is 
different  for  each  individual  and  the  visibility  curve  usually  has 
the  general  form  shown  in  Fig.  88  as  given  by  Nutting  for  sev- 
eral observers  and  reduced  to  a  common  maximum. 

In  view  of  their  general  utility  in  the  application  of  optical 


256 


HIGH   TEMPERATURES 


pyrometers  to  the  measurement  of  the  temperatures  of  exposed 
surfaces,  we  give  in  the  Appendix,  Table  X,  the  values  of  the 
emissivities  or  absorption  coefficients  (a  =  e/e),  where  e  =  i,  for 
certain  opaque  substances  as  determined  by  various  observers; 
and  if  the  assumption  is  made  in  all  cases  that  a  has  no,  or  a  very 
small,  temperature  coefficient,  the  above  equations  may  be  used 
to  compute  the  true  temperatures  of  such  substances  when  their 
absorbing  power  a  is  known,  as  well  as  their  apparent  tempera- 


i.o 


0.5 


.\ 


Wave  Length  450  500  550 

Fig.  88.    Visibility  Curve  of  Eye. 

tures  as  given  by  an  optical  or  radiation  pyrometer,  remember- 
ing that  the  above  formulae  apply  only  when  expressed  in  degrees 
absolute  (/ +  273°  C.). 

We  may  illustrate  by  a  numerical  example  the  use  of  Table  X 
for  the  estimation  of  the  true  temperature  of  a  metal  from  obser- 
vations with  an  optical  pyrometer,  using  red  light  of  wave  length 
X  =  0.65  /*.  Let  us  take  the  case  of  the  pouring  of  a  stream  of 
iron  whose  surface  is  clear,  and  suppose  the  optical  pyrometer 
reading  to  be  1427°  C.  when  the  pyrometer  is  sighted  on  the  clear 
stream.  From  Table  X,  the  value  of  the  absorption  coefficient 
a  for  iron  at  X=  0.65  /*  is  0.415.  In  equation  (Illb),  we  have  to 
solve  for  T  as  follows: 


1427  +  273 


0.65  •  log  0.415 
14,500  •  0.4343 


THE   LAWS   OF   RADIATION 


257 


whence  /=  I1— .273  =  1549°   C.,    the  true  temperature   of   the 
stream  of  metal. 

Equation  (IHb)  may  also  be  solved  graphically  as  shown  in 
Fig.  89,  for  pyrometers  using  red  light,  in  which  each  of  the 


T- 


MOO 


900 


800 


700 


=0.p5/ 


CORRECTIONS  TO 

OPTICAL  PYROMEfER 

FOR  ABSORBING  POWERS 


FOR  RED  LIGHT  OF 
WAVE  LENGTH=0.65M 

(a=e=i-r) 


7 


a- 


o.u 


0.2 


a  =0.3  X" 


M 


100 
°C 


0          700  °C    900        1100      1300       1500       1700       1900       2100       2300       2500       2700     2900 
Optical  Pyrometer  Readings,  Centigrade 

Fig.  89.     Corrections  to  Optical  Pyrometer. 

curves  represents  a  given  absorbing,  emissive,  or  reflecting  power 
(a  =  e  =  i  —  r),  the  abscissae  are  pyrometer  readings  in  centi- 
grade, and  the  ordinates  the  corresponding  corrections  to  be 
added  to  the  pyrometer  readings  for  substances  of  known  emis- 
sive powers.  Similar  graphs  could  be  constructed  for  pyrometers 
using  other  than  red  light.  Methods  of  graphical  reduction  have 


HIGH  TEMPERATURES 


1800  -- 


.1900  -- 


2100  -- 


2300  -- 


2400-  - 


m   1250r 

„_ 

-2000 

p 

3300 

-- 

- 

- 

• 

3200 

_ 

- 

: 

:  _ 

3100 

- 

~ 

-1900 

"  ~_      1300  - 

: 

_ 

- 

"" 

3000 

-- 

-0,01 

- 

-- 

2900 

--      1350  - 

_ 

-0,02 

- 

-1800 

-- 

2800 

~- 

- 

-0,03 

- 

-- 

2700 

~  ~ 

- 

-0,04 

- 

- 

: 

-0,05 

~ 

" 

.1      1400  - 

XXXN             = 

-  0,06 
-  0,07 

- 

-1700 

:  — 

2600 

: 

xx                    ~ 

-0,08 
-0,09 

- 

:- 

_ 

\        — 

-0,1 

— 

x; 

• 

:" 

2500 

.  I      1450  - 

~- 

-0,2      XV 

- 

- 

- 

xx 

- 

-- 

2400 

-'- 

_ 

-  0,3                       ^x 

- 

-1600 

- 

- 

-0,4 

.  :    isoo  - 

-0,5 

' 

2300 

- 

- 

-0,6 

'. 

- 

— 

-o,r 

~ 

_  - 

-0,8 

— 

~ 

— 

-0,9 

- 

; 

PS 

: 

--      1550  - 

- 

" 

2200 

-  = 

• 

-1500 

:- 

-  :   ISM- 

- 

- 

•- 

2100 

S'  167°  < 

P 

T 

] 

T' 

2000 

Fig.  go.    Pirani's  Method. 


THE  LAWS  OF  RADIATION  259 

also  been  described  by  Wartenberg,  who  plots  lines  of  tempera- 
ture readings  with  absorption  coefficients  as  ordinates  and  tem- 
perature corrections  as  abscissae;  and  by  Pirani,  who  has  devised 
a  protractor  or  vernier  method  for  the  solution  of  Wien's  equa- 
tion, reproduced  in  Fig.  90,  in  which  a  line  drawn  from  any 
observed  temperature  S  or  Si  through  the  value  a  of  the  absorp- 
tion coefficient  for  the  substance  observed  intersects  the  lines  T 
or  Ti  at  the  point  corresponding  to  the  true  temperature.  This 
figure  is  constructed  for  X=  0.65  M  and  cz  =  14,200.  To  trans- 
form the  value  of  a  to  the  basis  of  any  other  value  of  c2,  as  14,500, 
use  may  be  made  of  the  equation  a  log  a  =  log  a',  where  a  is  the 
ratio  of  the  c's.  (See  also  Table  IX  in  the  Appendix.) 

It  should  be  noted,  in  making  use  of  the  figures  given  in  Table 
X,  that  they  apply,  with  some  exactness,  in  the  case  of  the 
metals,  to  bright  or  polished  surfaces  only  of  either  the  solid 
or  liquid.  When  rough  surfaces  are  met  with,  the  values  of  the 
absorbing  factors  should  in  general  be  increased  considerably 
over  those  given  in  the  table.  Also,  these  last  are  uncertain  for 
a  single  metal,  in  some  cases  by  as  much  as  10  per  cent,  as  deter- 
mined by  different  observers,  due  in  large  part,  apparently,  to 
the  varying  degree  of  polish  of  the  samples  used.  Also,  it  should 
be  noted  that  the  surfaces  of  many  substances  undergo  changes 
of  emissivity  on  continued  heating. 

The  expression  for  the  emissivity  Ex  of  total  radiation,  to  use 
with  radiation  pyrometers  based  on  Stefan's  law,  is: 

log  Ex=  4  (\ogT-logS) (Ba) 

There  is  little  data  on  the  total  emissivity  E  for  either  the 
metals  or  other  substances  of  interest  in  the  measurement  of 
temperatures  by  means  of  total-radiation  pyrometers.  A  rough 
idea  of  the  order  of  the  value  of  the  total  emissivity  E  (equa- 
tion (Ba))  is  given  by  an  examination  of  both  the  visible 
and  infra-red  values  of  the  absorbing  powers.  The  values  at 
2,  5,  and  SM  are  given  in  Table  X.  Thus  for  iron,  the  average 
value  of  a  from  the  table  is  0.28,  and  the  observed  value  of  E  is 
0.29.  The  infra-red  values  of  a  as  w.ell  as  those  of  E,  in  general, 


260  HIGH  TEMPERATURES 

vary  with  the  temperature,  which  fact  renders  their  exact  deter- 
mination for  high  temperatures  a  difficult  operation.  Deter- 
minations of  E  at  high  temperatures  have  been  made  for  liquid 
copper,  0.14;  iron,  0.29;  cuprous  oxide,  0.60. 

There  is  room  for  a  great  deal  of  experimental  work  in  deter- 
mining satisfactorily  the  emissive  properties  at  high  temperatures 
of  those  substances  met  with  in  pyrometric  practice. 


CHAPTER  VII. 
RADIATION  PYROMETER. 

Principle.  —  The  quantity  of  heat  a  body  receives  by  radiation 
from  another  body  depends  on  certain  conditions  relative  to  each 
of  the  two  bodies,  which  are: 

i  .   Temperature  ; 

2.  Area  of  surface; 

3.  Distance  apart; 

4.  Emissive  and  absorbing  power. 

In  order  to  utilize  heat  radiation  for  the  determination  of  tem- 
peratures, one  measures  a  heat  change  produced  on  the  object 
used  as  an  instrument  by  the  object  to  be  studied;  this  heat 
change  is  either  a  rise  of  temperature  or  a  resulting  phenomenon, 
such  as  a  change  of  electrical  resistance,  thermoelectromotive 
force,  expansion,  etc. 

The  quantity  of  heat  given  off  is  proportional  to  the  area  of 
the  radiating  surface  S,  and  varies  inversely  as  the  square  of  the 
distance  /. 


d  being  the  diameter  of  the  radiating  surface  and  E  its  emissive 
power. 

Now,  -  is  the  apparent  diameter  of  the  object;  the  quantity 
/ 

of  heat  radiated  depends,  then,  upon  the  solid  angle  under  which 
the  object  is  seen.  Any  instrument  making  use  of  the  intensity 
of  radiation  must,  therefore,  have  a  receiving  device  of  sufficiently 
small  area  so  that  it  may  be  completely  covered  by  the  desired 
radiation. 

The  emissive  power  E  is  very  variable  from  one  substance  to 
another,  as  we  have  seen,  and  for  the  same  substance  variable 

261 


262  HIGH  TEMPERATURES 

with  the  temperature.  It  would  be  desirable  to  determine  this,, 
but  that  is  difficult  and  often  impossible,  especially  at  high  tem- 
peratures, although  some  small  advance  has  been  made  in  this 
direction  for  a  few  substances,  as  we  have  seen  in  the  preceding 
chapter. 

The  coefficient  k"  is  a  function  of  the  temperature  alone,  which 
expresses  the  law  of  variation  of  the  radiation  with  the  tempera- 
ture. This  law  should  be  determined  in  the  first  place.  It  is 
on  the  more  or  less  exact  knowledge  of  this  law  that  the  entire 
accuracy  of  the  results  depends.  We  have  seen  that  Stefan's 
law  (page  245)  satisfies  all  requirements,  at  least  in  the  case  of 
*  black  bodies,  for  the  measurement  of  total  radiation,  although  the 
early  experimenters,  working  before  the  establishment  of  this 
law,  were  obliged  to  express  their  results  empirically,  and  great 
confusion  resulted  from  the  different  assumptions  made. 

Early  Investigators:  Temperature  of  the  Sun.  —  Let  us  see 
now  what  are  the  experimental  arrangements  which  have  been 
used  ta  measure  the  intensity  of  heat  radiation;  these  earlier 
measurements  had  for  their  only  aim  the  determination  of  the 
sun's  temperature. 

Later  were  developed  the  more  elaborate  and  sensitive  types 
of  apparatus  which  were  suitable  also  for  laboratory  investiga- 
tions, and  which  were  used,  for  example,  in  the  experimental  dem- 
onstration of  the  laws  of  radiation;  and  finally  we  have  to-day 
several  total-radiation  pyrometers  of  simple  construction  which 
are  of  great  usefulness  both  as  industrial  and  as  scientific  instru- 
ments. 

We  shall  consider  in  order  the  above-mentioned  aspects  of  this 
development  of  total-radiation  methods,  noting  that  the  early 
observers  labored  under  a  threefold  handicap, — lack  of  knowledge 
of  the  temperature  scale,  of  the  radiation  laws,  and  lack  of  suffi- 
ciently sensitive  instruments. 

PouilleCs  Pyrheliometer.  —  Before  Pouillet,  Gasparin  had  al- 
ready made  some  trials.  His  apparatus  consisted  of  a  hollow 
brass  sphere  mounted  on  a  foot  and  blackened;  "a  thermometer 
was  used  to  measure  the  rise  in  temperature  of  the  water  contained 


RADIATION  PYROMETER 


263 


in  the  sphere.     The  advantage  of  this  arrangement  was  that  the 
apparatus  was  always  turned  properly  toward  the  sun.  , 

The  pyrheliometer  of  Pouillet  consists  of  a  calorimeter  .which 
measures  directly  the  heat  received  by  radiation  (Fig.  91).  A 
very  thin  silver  box  is  carried  by  a  hollow  tube,  cut  along  a  gener- 
atrix to  let  the  thermometer  be  seen.  The 
box  is  of  100  mm.  diameter  by  15  mm.  height; 
it  contains  100  c.c.  of  water.  At  the  lower 
part  of  the  box  is  located  a  metallic  disk  of 
the  same  diameter  as  the  box,  and  serving 
to  turn  the  apparatus  toward  the  sun;  it 
suffices,  in  fact,  for  the  shadows  of  the  box 
and  disk  to  coincide  exactly  in  order  that  the 
system  be  properly  pointed.  A  knob  serves 
to  turn  the  apparatus  about  its  axis  in  order 
to  stir  the  water.  Finally  a  support  gives 
the  means  of  placing  the  system  in  any  de- 
sired orientation. 

To  take  an  observation,  the  apparatus  is 
set  up  and  shielded  from  the  sun's  action  by 
means  of  a  screen;  the  readings  of  the  ther- 
mometer are  taken  for  five  minutes;  the 
screen  is  removed  and  the  thermometer  is 
read  for  five  minutes ;  the  screen  is  put  back, 
and  a  new  set  of  readings  of  the  thermometer  for  five  minutes 
is  taken. 

The  first  and  the  third  sets  furnish  the  corrections  due  to  the 
surroundings.  Pouillet  observed  in  this  way  a  rise  of  tempera- 
ture of  one  degree  in  five  minutes. 

In  the  determination  of  the  temperature  of  the  sun,  it  was 
evidently  necessary  to  take  into  account  the  heat  absorbed  by 
the  atmosphere  (it  is  about  20  per  cent  of  the  total  radiation 
from  the  sun).  Pouillet  found  by  this  method  1306°  for  the 
temperature  of  the  sun. 

V wile's  Actinometer.  —  The  principle  of  this  apparatus  is  quite 
different  from  that  of  the  preceding;  one  observes  the  stationary 


Fig.  91.     Pouillet's 
Pyrheliometer. 


264 


HIGH  TEMPERATURES 


equilibrium  of  a  thermometer  receiving  simultaneously  radiation 
from  an  inclosure  at  fixed  temperature,  and  that  from  the  hot 
substance  to  be  investigated  (Fig.  92). 

The  apparatus  consists  of  two  spherical  concentric  coverings 
of  brass,  in  which  a  water  circulation  may  be  set  up  at  constant 
temperature,  or  ice  may  be  substituted  for  water.  The  inner 
covering  of  150  mm.  diameter  is  blackened  inside.  The  ther- 
mometer has  a  spherical  bulb  whose  diameter  varies  from  5  to 

15  mm.;  the  surface  of  the  bulb  is 
also  blackened.  The  scale  is  divided 
into  fifths  of.  a  degree.  The  entrance 
tube  carries  a  diaphragm  pierced  with 
holes  of  different  diameter;  on  the 
extension  of  this  tube  is  located  an 
opening  closed  by  a  ground-glass 
mirror  slightly  blackened,  which  per- 
mits of  determining  that  the  solar 
rays  fall  quite  exactly  upon  the  ther- 
mometer bulb. 

The  establishment  of  the  tem- 
perature equilibrium  requires  fifteen 
minutes,  and  the  differences  of  tem- 
perature observed  vary  from  15° 
to  20°. 


Fig.  92.     Violle's  Actinometer. 


Violle  found  in  this  way,  for  the  temperature  of  the  sun,  figures 
varying  from  1500°  to  2500°. 

Pouillet  and  Violle  made  use  of  Dulong  and  Petit's  law  of 

radiation, 

q  =  al, 

that  the  discoverers  had  established  by  observations  reaching 
only  to  300°. 

The  constant  a  may  be  determined  for  each  apparatus  by  a 
single  experiment  made  at  a  known  temperature.  This  law,  as 
we  shall  show  farther  on,  is  not  exact,  so  that,  according  to  the 
temperature  used  to  determine  the  constant,  a  different  value  of 
the  latter  is  found,  and  consequently  also  different  values  at 


RADIATION   PYROMETER 


265 


temperatures  calculated,  assuming  this  law  to  hold.  This  is  the 
reason  for  the  differences  between  the  three  figures,  1300,  1500, 
and  2500,  of  Pouillet  and  Violle.  They  correspond  to  determi- 
nations of  the  constant  obtained  by  means  of  preliminary  experi- 
ments made  at  the  temperatures  of  100°,  300°,  and  1500°. 
The  elder  Secchi,  making  use  of  Newton's  formula, 


still  more  inexact,  found  for  the  sun's  temperature  several  mil- 
lions of  degrees. 

Work  of  Rosetti.  —  The  Italian  scientist,  Rosetti,  was  the  first 
to  grasp  the  fundamental  importance  of  the  choice  of  the  law 


Fig.  93.     Rosetti 's  Apparatus. 

assumed  for  radiating  power;  he  showed  that  a  graduation  made 
by  an  experiment  at  300°  gave  for  the  temperature  of  a  body 
heated  in  the  oxy hydrogen  flame: 

46,000  if  one  uses  the  law  of  Newton; 
1,100  if  one  uses  the  law  of  Dulong  and  Petit. 

Now  the  temperature  of  the  oxyhydrogen  flame  is  about  2000°. 

This  physicist  used  a  thermoelectric  pile  whose .  sensibility 

could  be  changed  without  touching  the  element;  in  the  apparatus 

of  Violle  it  is  necessary,  on  the  contrary,  to  change  the  thermom- 


266  HIGH  TEMPERATURES 

eter,  a  proceeding  which  renders  the  observations  comparable 
with  difficulty. 

The  pile  (Fig.  93)  consists  of  twenty-five  sheets  of  bismuth  and 
antimony;  these  sheets  are  very  thin,  for  the  whole  of  the  appa- 
ratus is  but  5  mm.  on  a  side.  The  whole  is  inclosed  in  a  small 
metallic  tube. 

To  make  an  experiment,  there  is  placed  before  the  pile  a  screen 
filled  with  water,  which  is  removed  at  the  instant  of  taking  an 
observation. 

A  preliminary  calibration  made  with  a  Leslie's  cube  of  iron 
filled  with  mercury  that  is  heated  from  o°  to  300°  gave  the  follow- 
ing results: 

Excess  of  the  temperature  of  Reading  of 

the  cube  over  the  surrounding  galvanometer, 

temperature. 

32.8° 10.0° 

112.8   55.0 

IQ2.8    I4I-9 

272.8    283.5 

Newton's  law  and  that  of  Dulong  and  Petit  giving  no  con- 
cordance between  the  numbers  observed  and  those  computed, 
Rosetti  proposed  the  formula 

Q  =  aT2(T-0)  -b(T-8), 

where  T  —  absolute  temperature  of  the  radiating  body,  0  =  the 
absolute  temperature  of  the  surroundings.  This  formula  with 
two  parameters  permits  necessarily  a  closer  following  of  the 
phenomenon  than  a  formula  with  but  a  single  parameter. 

T_a  Deflections  Deflections  computed. 

observed.  Dulong's  law.          Rosetti's  law. 

SO      ^ 
IOO 

150 

2OO 
250 

Rosetti  showed  later  that  the  formula  he  proposed  did  not 
lead  to  absurd  results  for  higher  temperatures.  A  mass  of  copper 
was  heated  to  redness  in  a  flame,  and  the  temperature  was  esti- 
mated by  the  calorimetric  method  (a  quite  uncertain  method,  as 


17-2 

A-\-2  .  12 

.4—0.23 

46.4 

+Q-95 

90.1 

—  2.12 

+0.70 

I5I-7 

+4-82 

+0.99 

234-7 

+  2.83 

—  0.12 

RADIATION  PYROMETER  267 

the  variation  of  the  specific  heat  of  copper  was  not  known).  The 
two  methods  gave  respectively  735°  and  760°.  This  difference 
of  25°  is  less  than  the  experimental  uncertainties. 

Disks  of  blackened  metal  placed  in  the  upper  part  of  a  Bunsen 
flame  gave,  according  to  the  formula,  temperatures  of  the  order 
of  1000°;  oxy chloride  of  magnesium  in  the  oxyhydrogen  blast 
lamp  gave  2300°.  All  these  numbers  are  possible. 

Rosetti,  using  this  formula,  found  10,000°  for  the  temperature 
of  the  sun,  this  figure  resulting  from  an  extrapolation  above  300°. 

Modern  Radiometric  Apparatus.  —  The  principles  most  often 
made  use  of  in  modern  receiving  apparatus  for  thermal  radiation 
are: 

1.  The  generation  of  an  electric  current  in  a  circuit  composed 
of  two  dissimilar  metals  by  the  radiation  which  falls  on  one  or 
more  junctions,  or  the  thermopile  of  Nobili,  often  called  the 
Melloni  thermopile,  which  we  have  just  seen  was  used  by  Rosetti, 
and  which  has  been  in  use  for  nearly  a  century,  and  in  recent 
years  rendered  very  sensitive. 

2.  The  increase  in  resistance  of  a  metallic  strip,  forming  one 
or  more  branches  of  a  Wheatstone  bridge,  due  to  the  rise  in  tem- 
perature caused  by  the  incident  radiation,  or  the  bolometer  of 
Langley. 

3.  The  deflection,  by  radiation,  of  vanes  delicately  mounted 
in  vacuo,  —  the  radiometer  of  Crookes. 

There  have  been  modifications  and  improvements  in  all  of 
these  types  of  apparatus,  some  of  which  we  shall  mention  briefly, 
such  as  the  combination  of  the  thermopile  and  moving-coil  galva- 
nometer known  as  the  radiomicrometer,  due  to  d'Arsonval  and 
Boys  independently.  They  can  all  be  used  to  measure  tempera- 
tures when  once  calibrated  in  terms  of  black-body  radiation, 
with  the  limitations  already  described  in  the  preceding  chapter 
or  applications  of  the  radiation  laws. 

It  should  perhaps  also  be  emphasized  at  this  point  that  for  a 
strict  application  of  the  laws  of  radiation  of  the  black  body  to 
such  apparatus,  not  only  the  radiation  source  but  also  the  re- 
ceiver should  be  black.  Plane  surfaces  covered  with  lamp-  or 


268  HIGH  TEMPERATURES 

platinum-black  are  an  approximation  to  this  condition,  which 
may  be  still  more  closely  realized,  when  possible  instrumentally, 
by  making  the  receiver  conical,  as  done  by  Fery;  or  better,  a  hol- 
low sphere  with  small  opening,  as  attempted  by  Mendenhall;  or 
by  inclosing  the  sensitive  portion  of  the  receiving  apparatus 
within  a  sphere  brightened  on  the  inside,  as  was  done  by  Paschen; 
or  finally,  within  a  diaphragmed  cylinder,  such  as  used  by  Langley 
and  Abbot.  All  of  these  forms  of  apparatus  are  rendered  more 
sensitive  by  mounting  in  vacuo,  and  the  radiometer  can  hardly 
be  used  otherwise ;  but  there  ensues  the  complication  of  the  selec- 
tive absorption  of  the  window,  which  may  become  serious  when 
extrapolating  for  high  temperatures,  especially  if  total  radiation 
is  used.  The  radiation  of  a  narrow  spectral  band  may  also  be 
used  with  all  such  apparatus,  but  this  cuts  down  the  sensibility 
enormously,  and  this  method  is  hardly  practicable  for  tempera- 
ture measurements,  except  perhaps  in  very  special  cases  in  the 
laboratory. 

Very  little  work  has  been  done  until  recently  with  radiometric 
apparatus  on  the  estimation  of  terrestrial  temperatures,  due  per- 
haps to  the  existence  of  other  methods  of  sufficient  accuracy  and 
sensibility;  the  radiometric  methods  being  in  practice  largely 
reserved  for  investigation  of  the  radiation  characteristic  of  various 
substances  at  high  temperatures  and  for  the  estimation  of  the 
temperature  and  spectral  radiation  of  the  sun,  especially  in  the 
infra-red.  The  possibility  of  rendering  the  radiometric  methods 
recording  and  the  recent  development  of  simple  types  of  appara- 
tus have  given  an  impetus  to  their  use  in  temperature  measure- 
ments. As  to  the  ultimate  sensibility  attainable  with  the  various 
types  of  apparatus  mentioned  above,  comparative  examination 
by  Coblentz  has  shown  that  there  is  very  little  choice  in  the 
matter,  although  each  apparatus  has  its  characteristics  which 
render  it  more  fit  in  certain  classes  of  problems  than  the  others. 

The  Thermopile.  —  This  instrument  was  the  earliest  to  be  used 
in  radiometric  work,  and  its  principle  is,  as  we  shall  see,  the  only 
one,  of  those  above  mentioned,  actually  used  in  the  construction 
of  an  instrument  primarily  designed  for  high-temperature  meas- 


RADIATION  PYROMETER 


269 


urements  radiometrically.  Types  of  multiple  thermopiles  are 
illustrated  in  Fig.  94.  In  a  is  shown  the  linear  thermopile  of 
Rubens  of  20  junctions  of  i  mm.  constantan  of  wire  about  o.i  mm. 
diameter  with  an  exposed  area  of  about  0.8  by  20  mm.  The  re- 


Fig.  94.    Multiple  Thermopiles. 

sistance  can  be  cut  down  by  shortening  the  connecting  wires  and 
the  heat  capacity  and  conduction  lowered  by  using  thinner  wires 
of  say  0.06  mm.,  and  by  making  the  unexposed  junctions  smaller 
than  the  others.  These  modifications,  shown  in  &,  are  due  to 
Coblentz.  For  sources  of  small  area  the  form  c  may  be  used. 
A  sensitive  galvanometer  is  required  whose  best  resistance  for 
highest  sensibility  is  that  of  the  thermopile. 

Callendar  has  recently  suggested  several  radio-balances  for 
measuring  radiation  in  absolute  measure,  which  are  suitable  in 
certain  forms  for  high-temperature  measurement.  Among  these 
is  his  disk  radio-balance  (Fig.  95),  in  which  heat  supplied  by 
radiations  is  directly  compensated  by  the  Peltier  absorption  of 
heat  in  a  thermo junction,  1 ,  through  which  a  measured  electric 
current  is  passed.  In  the  simplest  form  of  the  instrument,  radia- 
tion admitted  through  a  measured  aperture  2  mm.  in  diameter 
falls  on  a  small  copper  disk  3  mm.  in  diameter  by  0.5  mm.  thick, 
to  which  two  thermojunctions  are  attached,  forming  a  Peltier 
cross.  One  couple  is  attached  to  a  sensitive  galvanometer  G  for 


270 


HIGH  TEMPERATURES 


indicating  changes  of  temperature.  The  other  is  connected  to  a 
battery  B  and  rheostat  R  in  series  with  a  milliammeter  or  poten- 
tiometer for  measuring  the  current  required  to  reduce  the  deflec- 
tion of  the  galvanometer  to  zero.  If  A  is  the  area  of  the  aperture 


Fig.  95.     Callendar's  Radio-balance. 

in  cmT2,  H  the  intensity  of  radiation  received  in  watts  per  crrT2, 
a  the  absorption  coefficient  of  the  surface  of  the  disk,  P  the 

7   771 

Peltier  effect  in  volts  (P  =  T  —  when  T  =  absolute  tempera- 

dE 
ture  and  —  the  thermoelectric  power),  C  the  balancing  current 

in  amperes,  and  R  the  effective  resistance  of  the  couple,  the 
equation  giving  the  value  of  the  radiation  in  absolute  measure  is 

a  A  H  =  PC  -  C2R. 

The  value  of  R  in  the  small  correction  term  for  the  joule  effect 

p 

is  readily  determined  by  observing  the  neutral  current  C0  =  — , 

R 

for  which  the  joule  effect  balances  the  Peltier  effect.  In  prac- 
tice, two  similar  disks  with  similar  connections  are  mounted  side 
by  side  in  a  thick  copper  box,  and  are  balanced  against  each  other 
in  order  to  avoid  changes  of  zero  due  to  exposure  to  stray  radia- 
tion, sunshine,  or  to  rapid  variations  of  temperature.  For  the 


RADIATION   PYROMETER 


271 


measurement  of  temperature  with  such  an  instrument,  H  may 
be  expressed  in  terms  of  the  emissivity,  distance,  and  temperature 
of  the  source,  or  the  instrument  may  be  calibrated  empirically 
by  means  of  a  black  body. 

The  Radiomicrometer .  —  We  may  illustrate  the  use  of  this- 
instrument  for  temperature  measurements  by  describing  the 
experiments  of  Wilson  and  Gray.  These  physicists  measured 


AM 


R' 


Fig.  96.     The  Radiomicrometer. 


the  intensity  of  radiation  by  means  of  a  thermoelectric  couple,  a 
method  first  conceived  by  Deprez  and  d'Arsonval.  A  movable 
coil  made  of  two  different  metals  (silver  and  palladium)  is  sus- 
pended by  a  silk  cocoon  fiber  between  the  poles  of  a  magnet. 
The  solar  radiation  is  allowed  to  fall  upon  one  of  the  junctions, 
while  upon  the  other  junction  is  directed  a  source  of  heat  which 
exactly  balances  the  first.  As  the  temperature  of  this  auxiliary 
source  is  necessarily  the  lesser,  it  is  necessary  that  the  apparent 
angle  which  it  subtends  at  the  galvanometer  be  the  greater. 


272  HIGH  TEMPERATURES 

Wilson  and  Gray  used  an  apparatus  similar  to  the  radiomi- 
crometer  of  Boys.  The  suspending  fiber  is  of  quartz;  the  metals 
employed  are  bismuth  and  antimony.  The  electromotive  force 
so  produced  is  twenty  times  greater  than  that  obtained  with  the 
palladium-silver  couple.  The  metallic  strips  R  and  R'  (Fig.  96) 
are  very  thin  (o.i  mm.),  which  renders  the  construction  of  the 
apparatus  quite  delicate.  In  order  to  protect  the  movable  coil 
against  air  currents,  it  is  inclosed  in  a  metallic  case;  an  open 
tube  lets  pass  in  the  radiation;  diaphragms  set  inside  this  tube 
prevent  air  disturbances. 

Instead  of  measuring,  as  may  be  done,  the  deflection  of  the 
mobile  parts,  these  observers  preferred  to  employ  a  null  method 
making  use  of  another  radiation,  that  from  a  modification  of  the 
meldometer  of  Joly,  an  apparatus  used  also  for  the  calibration  of 
the  radiomicrometer.  The  meldometer  (Fig.  128)  consists  of  a 
strip  of  platinum  heated  by  an  electric  current;  the  dimensions  are 
as  follows:  102  mm.  in  length,  12  mm.  in  breadth,  and  o.oi  mm. 
thick.  This  strip  they  placed  in  the  midst  of  an  inclosure  sur- 
rounded by  water.  Fastened  at  one  end,  it  is  held  in  place  at 
the  other  end  by  a  spring  and  carries  on  this  end  a  lever  to  which 
is  fixed  a  mirror  arrangement  serving  to  optically  amplify  the 
variations  in  the  length  of  the  strip  resulting  from  its  heating  by 
the  passage  of  the  more  or  less  intense  current. 

The  relation  between  the  change  of  length  and  the  temperature 
is  determined  by  means  of  the  fusion  of  very  small  fragments 
(YO  milligram)  of  bodies  whose  fusing  points  are  known.  Wilson 
and  Gray  used  the  following,  which  for  the  gold  and  palladium 
are  certainly  too  low: 

Silver  chloride 452  ° 

Gold 1045 

Palladium 1500 

With  this  apparatus  they  apparently  verified,  up  to  the  fusion 
of  platinum,  the  law  of  radiation  given  by  Stefan: 

E  =  *(T*-  To4). 

For  the  purpose  of  graduation,  the  meldometer  was  removed 
to  a  distance,  so  that  its  action  on  the  radiomicrometer  was 


RADIATION   PYROMETER  273 

always  the  same,  and  it  was  assumed  that  the  intensity  varies  as 
the  inverse  square  of  the  distance.  It  is  besides  necessary  to 
know  the  emissive  power  of  platinum;  Wilson  and  Gray  took  as 
starting  points  the  results  given  by  previous  experiments: 

t°  Emissive  power. 

^ 


800 


3-9 


And  by  extrapolation  they  found  —  at  the  temperature  of  1250°, 

2.9 

the  temperature  which  balanced  the  solar  radiation  with  the  some- 
what large  apparent  angle  subtended  by  the  meldometer.  In 
admitting,  then,  with  Rosetti  and  Young,  a  zenith  absorption  of 
30  per  cent,  the  temperature  of  the  sun,  supposed  to  be  a  black 
body,  was  found  equal  to  about  5900°  C. 

This  figure,  although  reasonable  numerically  in  the  light  of 
later  work,  must  be  considerably  uncertain,  on  account  of  the 
errors  involved  in  the  fusing  points  employed  for  graduation, 
and  because  of  the  fact  that  the  radiation  from  platinum  does 
not  obey  Stefan's  law.  Furthermore,  the  constants  for  platinum 
were  found  in  terms  of  those  of  copper  oxide,  a  substance  they 
found,  incorrectly,  to  depart  more  from  a  black  body  than  pol- 
ished platinum. 

Wilson  has  also  given  5500°  C.  as  the  best  result  from  his  own 
experiments,  using  a  black  body  as  comparison  source.  Wilson 
and  Gray  also  found  the  temperature  of  the  carbon  arc  to  be  3330° 
C.,  a  result  now  known  to  be  considerably  low  (see  Chap.  XI). 

The  Bolometer.  —  Although  the  principle  of  measuring  the 
intensity  of  radiation  by  the  change  in  resistance  of  a  metallic 
strip  had  been  used  by  several  observers  before  Langley,  he 
nevertheless  deserves  the  credit  for  first  constructing  a  practical 
instrument  and  developing  it  to  a  very  high  state  of  sensibility. 
There  have  been  several  types  of  bolometer  used,  although  in  all 
of  them  the  Wheatstone  bridge  method  of  measuring  resistance 


274  HIGH  TEMPERATURES 

t 

is  employed.  For  spectrophotometric  work,  usually  two  narrow 
strips  of  extremely  thin  platinum  serve  as  adjacent  arms  of  the 
bridge;  in  the  Smithsonian  instrument  used  by  Langley  and 
Abbot,  the  strips  are  12  mm.  long,  0.06  mm.  wide,  and  so  thin 
that  the  resistance  is  about  4  ohms;  the  measuring  current  is 
0.03  milliamperes.  The  measurements  are  the  same  as  those 
with  the  resistance  thermometer  used  with  a  bridge,  and,  in 
special  work,  an  extremely  sensitive  galvanometer,  usually  a 
Kelvin  multiple-coil  instrument  of  low  resistance,  short  period, 
and  the  highest  possible  current  sensibility,  io~10  to  5.io~u 
amperes  per  mm.  at  i  m.  A  spectrometer  of  proper  design  is  of 
course  also  essential  in  spectral  radiation  work. 

For  measurement  of  the  total  energy,  the  grid  form,  or  surface 
bolometer,  such  as  used  by  Lummer  and  Kurlbaum  in  their 
verification  of  the  radiation  laws,  is  preferable.  In  their  instru- 
ment, they  had  four  similar  bridge  arms  of  platinum  foil  composed 
of  12  connected  strips  32  by  i  mm.  and  o.ooi  mm.  thick;  two 
diagonal  arms  being  placed  one  behind  the  other  and  exposed  to 
the  radiation.  The  highest  temperature  sensitiveness  attained 
with  the  bolometer  is  io~7  deg.  C.  per  i  mm.  deflection.  The 
portion  of  the  instrument  receiving  radiation  is  inclosed  within 
a  well-screened  and  jacketed  case,  which  may  be  evacuated  if 
desired,  and  a  lens  or  mirror  used  for  concentrating  the  radiation. 

In  Callendar's  form  of  absolute  bolometer,  which  is  of  the  grid 
or  surface  type,  the  intensity  of  radiation  in  absolute  measure  is 
determined  by  observing  the  value  of  the  electric  current  required 
to  produce  the  same  rise  of  temperature  in  the  grid  as  the  radia- 
tion to  be  measured.  Callendar  has  also  introduced  several 
instrumental  improvements,  such  as  automatic  experimental  com- 
pensation of  that  part  of  the  grid  not  receiving  radiation  directly 
thus  eliminating  the  creep  in  attaining  a  maximum.  This  in- 
strument is  also  made  self-recording.  Callendar  has  also  de- 
veloped several  modifications  in  bolometer  design  that  may  prove 
serviceable  in  temperature  measurements,  particularly  when  a 
relatively  large  area  is  available. 

Although  the  bolometer  does  not  appear  to  have  been  used  as 


RADIATION   PYROMETER 


275 


a  pyrometer,  it  can  readily  be  adapted  for  that  purpose;  and  in- 
struments of  sufficient  sensitiveness  and  robustness  could  readily 
be  devised.  They  can  of  course  be  made  self -registering. 

The  Radiometer.  —  This  appears  to  be  the  least  adapted  for 
temperature  measurements  of  the  radiometric  instruments  we 
have  mentioned.  The  apparatus  consists  of  two  blackened  vanes 


5  cm 


Fig.  97.     Radiometer. 

hung  in  vacuo  on  a  fine  quartz  fiber  (Fig.  97).  Radiation  falling 
on  a  vane  deflects  the  suspended  system,  whose  angle  is  read,  as 
in  the  case  of  a  galvanometer,  with  mirror,  scale,  and  telescope. 
The  readings  are  influenced  by  many  factors,  notably  by  the  resid- 


276  HIGH  TEMPERATURES 

ual  gas  pressure,  the  location  of  the  vanes,  and  the  nature  of  the 
window.  The  instrument  is  not  transportable  and  cannot  be 
calibrated  in  absolute  measure.  Its  sensitiveness  is,  however, 
very  great. 

Standard  Pyrheliometers.  —  The  International  Union  for  Coop- 
eration in  Solar  Research  in  1905  adopted  temporarily  Angstrom's 
compensation  pyrheliometer  as  standard.  In  this  instrument, 
radiation  is  received  on  a^metallic  strip,  beside  which,  but  shielded 
from  the  radiation,  is  a  similar  strip  through  which  is  sent  a 
measured  electric  current  of  such  strength  that  the  temperatures 
of  the  two  strips  are  the  same,  as  measured  by  attached  thermo- 
couples. The  assumption  is  then  made  that  the  strip  carrying 
the  current  may  be  substituted  for  that  receiving  the  radiation. 

Calling  Q  the  heat  produced  in  gram  calories  per  minute  by 
the  current,  proportional  therefore  to  the  radiation  intensity,  r 
the  resistance  of  the  strip,  and  i  the  current,  we  have,  following 
Angstrom: 

^        60      ri  .« 

Q  = •  —  =  const.  -  j*. 

4.19     ba 

where  a  =  absorbing  power  of  blackened  strip  surfaces  and  b  = 
width  of  strips.  The  early  instruments  were  made  with  platinum 
strips,  but  Callendar  having  shown  that  the  temperature  coeffi- 
cient of  this  metal  is  a  serious  source  of  error,  strips  of  manganin 
are  now  used.  Angstrom  has  used  his  pyrheliometer  in  a  con- 
siderable number  of  laboratory  investigations,  including  a  study 
of  the  radiation  from  incandescent  lamps  and  from  the  Hefner 

standard.     The  latter  he  determined  to  radiate  — —  '  °         :- 

mm.    cm. 

As  used  by  Callendar,  Abbot,  and  others,  the  Angstrom  instru- 
ment, compared  with  absolute  radiometric  apparatus,  gives 
slightly  too  small  values  of  radiation  intensity. 

At  the  Astrophysical  Laboratory  of  the  Smithsonian  Institu- 
tion, Abbot  and  Fowle  have  recently  devised  an  absolute  radio- 
metric  instrument  composed  of  a  black-body  receiver  combined 
with  a  flow  calorimeter.  V.  A.  Michelson  in  1894  had  also  used 
a  black-body  receiver  combined  with  a  Bunsen  calorimeter.  A 


RADIATION   PYROMETER 


277 


form  of  the  Smithsonian  standard  pyrheliometer  is  illustrated 
in  part  in  Fig.  98,  in  which  a  is  the  diaphragmed  chamber  with 


conical  base  for  receiving  the  radia- 
tion which  first  passes  through  the 
blackened  tube  5,  also  supplied  with 
diaphragms  c,  c,  and  an  electro- 
magnetically  operated  shutter  gh. 
The  water  enters  at  e\  and,  after 
circulating  over  the  walls  d  and  / 
of  the  double  water  jacket,  passes 
out  at  62  into  an  automatic  weighing 
apparatus.  At  /i,  /2,  /3,  /4  are  the 
coils  of  platinum-resistance  ther- 
mometers giving  the  temperature  of 
the  water  before  and  after  absorb- 
ing the  radiation.  The  constants 
of  the  instrument  are  determined 
by  placing  a  heating  coil  at  m  and 
measuring  the  input  of  energy  elec- 
trically. It  was  found  that  the 
calorimeter  recorded  practically  100 
per  cent  of  the  energy  supplied  by 
the  heating  coil. 

Either  of  these  pyrheliometers 
could  be  used  to  measure  tempera- 
tures, and  it  is  not  impossible  that 
an  instrument  of  the  latter  type, 
or  of  Michelson's,  may  be  of  use  in 
absolute  pyrometric  apparatus  in 
those  researches  where  it  is  essen- 
tial that  the  receiver  be  a  black 
body  and  where  it  may  be  desired  to 
measure  the  radiation  in  absolute  units .  Secondary  pyrheliometers 
have  also  been  designed  recently  by  Abbot,  Marvin,  and  others. 

Thermoelectric   Telescopes.  —  The  Fery  pyrometer  was   the 
first  convenient  form  of  instrument  making  use  of  total  radiation 


278  HIGH  TEMPERATURES 

and  based  on  Stefan's  law  (page  245)  to  come  into  practical  use 
for  temperature  measurements.  As  in  the  case  of  the  photo- 
metric pyrometers,  the  limitations  as  to  the  realization  of  a 
black  body  apply  here  also  with  even  greater  emphasis,  as  an 
instrument  using  the  whole  spectrum,  visible  and  invisible,  is 
most  sensitive  to  selective  radiation  effects. 
Use  is  made  of  the  Stefan-Boltzmann  law, 


in  the  following  way:  Radiation  from  an  incandescent  body  is 
focused  upon  a  very  sensitive  thermocouple  and  raises  its  tem- 
perature. The  electromotive  force  thus  generated  at  the  junc- 
tion actuates  a  sensitive  potential  galvanometer  in  series  with 
the  couple  in  exactly  the  same  way  as  in  the  Le  Chatelier  thermo- 
electric pyrometer;  so  that  we  have  here  a  radiation  pyrometer 
which  is  direct-reading  by  means  of  a  pointer  on  a  scale,  and  may 
therefore  readily  be  made  a  recording  instrument. 

The  difficulty  in  construction  of  such  an  instrument  is  realizing 
a  material  for  lens  which  is  transparent  for  all  radiations  visible 
and  invisible,  so  that  the  pyrometer  may  be  calibrated  directly 
in  terms  of  Stefan's  law,  and  so  that  its  indications  will  be  reliable 
at  temperatures  however  high.  This  is  effected  in  the  laboratory 
type  of  instrument  by  use  of  a  fluorite  lens  which  for  tempera- 
tures above  900°  C.  satisfies  the  conditions  of  not  altering  appre- 
ciably the  radiations  transmitted  through  it;  that  is  to  say,  the 
ratio  of  the  radiations  absorbed  to  the  radiation  transmitted  is 
constant. 

At  low  temperatures  a  large  proportion  of  the  energy  exists 
in  the  form  of  long  wave  lengths,  and  as  fluorite  has  an  absorption 
band  in  the  infra-red  (near  6  /*),  it  will  absorb  a  considerable 
proportion  of  the  radiation,  and  therefore  Stefan's  law  can  no 
longer  be  assumed. 

Fig.  99  illustrates  the  construction  of  the  original  laboratory 
form  of  the  instrument,  where  F  is  the  fluorite  lens,  P  a  rack  and 
pinion  for  focusing  the  radiations  upon  the  thermojunction  of 
iron-constant  an,  and  protected  from  extraneous  rays  by  the 


RADIATION   PYROMETER 


279 


screens  C,  D,  shown  also  in  section  at  AB.  The  thermo junction 
is  of  exceedingly  small  dimensions,  only  a  few  thousandths  of  a 
millimeter  wide,  and  is 
soldered  to  a  silver  disk. 
The  leads  are  brought  out 
to  the  insulated  binding 
posts  b,  b',  so  placed  as 
to  reduce  the  chances  of 
extraneous  thermal  cur- 
rents to  a  minimum.  The 
circuit  is  completed 
through  a  sensitive  galva- 
nometer provided  with  a 
scale.  A  diaphragm  fixed 
in  size  and  position,  EE, 
gives  an  opening  of  con- 
stant angle  independent 
of  the  focusing,  whereby 
the  cone  of  rays  striking 
the  junction  is  not 
changed  in  size  by 
focusing. 

In  making  a  tempera- 
ture measurement,  it  is 
necessary  to  sharply  focus 
the  image  of  the  incan- 
descent object  upon  the 
thermo  junction  by  means 
of  the  eyepiece  O,  and  care 
must  be  taken  that  this 
image  is  of  greater  size 
than  the  junction.  This 
adjustment  once  made,  the 
pyrometer  operates  indefi- 
nitely while  sighted  upon  the  same  object,  and  readings  of  the  gal- 
vanometer scale  give  temperatures  directly  from  the  calibration. 


280  HIGH  TEMPERATURES 

The  precision  attainable  with  this  form  of  instrument,  over 
the  range  it  may  be  controlled  with  the  thermoelectric  pyrometer, 
is  shown  from  data  obtained  by  Fery,  assuming  Stefan's  law  to 
hold  in  the  form, 

CE  =  d  =  7.66  T4X  io-  12, 

where  E  is  the  total  energy  of  radiation  and  d  the  galvanometer 
deflection  and  T  the  absolute  temperature. 

,  Temperature  from    Temperature  from  A  in  Error 

thermocouple.  Stefan's  law.  degrees.  in  %. 

ii.  o  844°  860°  +16°  1.85 

14.0  914  925  +H  .84 

17.7  990  990  o  .o 

21.5  1054  1060  +6  .60 

26.O  II2O  II2O  O  .O 

32.2  1192  II9O  —  2  .17 

38.7  1260  1250  —io  .80 

45-7  J328  1320  -  8  .60 

52-5  1385  1380  -  5  .36  ,  , 

62.2  1458  1450  .50 

It  is  evident,  furthermore,  that  if  the  galvanometer  has  a 
uniformly  graduated  scale  and  the  temperature  TI  corresponding 
to  any  one  scale  reading  RI  is  known,  that  for  any  other  reading 
Rz  may  be  found  from  the  relation 


which  also  shows  that  errors  in  the  galvanometer  readings  are 
divided  by  four  when  reduced  to  temperatures.  For  very  high 
temperatures,  deflections  off  the  scale  of  the  galvanometer  will 
be  obtained  and  the  instrument  will  be  excessively  heated. 
Fery  overcomes  these  difficulties  by  substituting  a  smaller 
diaphragm  before  the  objective  when  the  radiation  is  reduced  in 
the  ratio  of  the  areas  of  the  apertures. 

The  highest  temperatures  which  may  be  estimated  by  this 
pyrometer  are  limited  only  by  the  applications  of  Stefan's  law 
to  this  extreme  region,  and  whether  Stefan's  law  applies  or  not, 
consistent  results,  nevertheless,  will  be  obtained. 

Instead  of  the  deflection  galvanometer,  it  is  better  in  work  of 
precision  to  substitute  a  low-range  potentiometer  with  sensitive 
galvanometer  (see  page  139). 


RADIATION   PYROMETER 


281 


The  laboratory  form  of  apparatus  described  above  is  not  suit- 
able for  use  in  technical  practice,  and  fluorite  is  difficult  to  get 
of  sufficient  size.  An  industrial  pyrometer  is  readily  made  by 
substituting  for  the  fluorite  lens  one  of  glass  of  wide  aperture, 
and  for  the  delicate  galvanometer  one  of  the  same  type  arid 
sensibility  as  used  in  thermoelectric  work;  the  resulting  instru- 
ment is  robust  and  sufficiently  sensitive  for  all  practical  uses,  and 
as  made  has  a  range  of  from  800°  to  1600°  C.,  although  the 
upper  limit  could  readily  be  extended  by  having  two  scales  on 
the  instrument,  provided  with  a  diaphragm. 

The  indications  of  the  industrial  form  of  this  pyrometer  will 
not  obey  Stefan's  law  exactly,  but  the  instrument  may  readily 
be  calibrated  by  direct  comparison  either  with  a  thermocouple 
or  with  a  laboratory  form  of  Fery  instrument,  and  the  scale  of 
temperatures  engraved  on  the  instrument.  Both  types  of  in- 
strument can  be  used  to  reach  lower  temperatures  (650°)  by 
means  of  more  sensitive  galvanometers. 


Fig.  100.     Fery  Mirror  Telescope. 

Fery  Mirror  Telescope.  —  This  instrument  (Fig.  100)  was  de- 
signed by  Fery  to  replace  both  the  laboratory  and  technical 
forms  of  lens  telescope,  and  has  been  used  very  considerably  in 
scientific  and  industrial  work.  As  usually  constructed  the  mirror 
is  of  gold  on  glass,  and  there  is  further  provided  an  ingenious 


282 


HIGH  TEMPERATURES 


optical  focusing  device  by  means  of  which  straight  lines  appear 
broken  (Fig.  101),  unless  the  instrument  is  in  focus.     The  range 


Fig.  101.     Focusing  Device. 

of  the  instrument  is  increased  by  means  of  a  sectored  diaphragm, 
so  that  temperatures  from  the  lowest  to  the  highest  may  be  read, 


Fig.  102.    Mounting  on  Kiln. 

although  for  reading  the  lowest  temperatures  with  any  consider- 
able precision  a  quite  sensitive  galvanometer  is  needed,  or  a  more 
sensitive  thermocouple  may  be  used  to  produce  the  same  effect. 


RADIATION   PYROMETER  283 

The  potentiometer  method  of  reading  for  very  accurate  work 
may  of  course  be  substituted,  as  with  the  other  forms  of  telescope. 
The  robustness  of  the  instrument  has  recently  been  increased  for 
industrial  work  by  substituting  pivot  galvanometers  for  the 
delicate  suspended-coil  instruments  hitherto  used.  The  gold 
mirror  may  be  considerably  tarnished  without  seriously  jn- 
fluencing  the  readings;  and  if  the  aperture  of  the  furnace  sighted 
upon  is  of  sufficient  size  and  the  telescope  in  focus,  the  tempera- 


Fig.  103.    Telescope  and  Galvanometer  in  Case. 

ture  readings  are  practically  independent  of  the  distance.  The 
instrument  takes  its  final  reading  very  promptly  with  only  slight 
creep.  The  readings  of  the  instrument  appear  to  be  influenced 
somewhat  by  the  area  sighted  upon  and  by  the  temperature  of 
the  region  immediately  outside  the  central  cone  of  rays,  or  in 
other  words,  by  stray  radiation.  For  example,  a  Fery  pyrom- 
eter, sighted  into  and  clear  through  a  resistance-tube  furnace  of 
75  mm.  aperture  open  its  whole  length  with  no  diaphragm, 
will  register  several  hundred  degrees  if  the  walls  of  the  furnace 
are  at  1100°  C.  An  aperture  of  about  i -inch- wide  opening  per 
i  yard  of  distance  (2.5  cm.  per  i  m.)  is  required  for  the  usual 
industrial  instruments.  A  suitable  mounting  for  determining 


284 


.HIGH  TEMPERATURES 


the  temperatures  of  a  kiln  is  shown  in  Fig.  102.  A  Fery  py- 
rometer packed  in  its  case  with  portable  galvanometer  mounted 
in  gimbals  is  shown  in  Fig.  103. 

Fery's  Spiral  Pyrometer.  —  Another  method  of  registering  the 
radiation  focused  by  the  telescope  mirror  has  been  devised  by 
Fery,  Fig.  104.  The  thermocouple  and  galvanometer  are  replaced 
by  a  bimetallic  spring  S  placed  at  the  mirror  focus,  carrying  an  alu- 
minium pointer  P  which  turns  over  a  dial  D  graduated  in  degrees 


Fig.  104.     Fery  Spiral  Pyrometer. 

of  temperature  in  response  to  the  differential  expansion  of  the 
spring  when  radiation  is  concentrated  upon  it.  This  instrument, 
therefore,  has  no  accessories,  and,  in  spite  of  a  zero  creep  and  set 
difficult  to  eliminate  in  the  spring,  this  form  of  instrument  may 
serve  satisfactorily  for  many  industrial  uses  where  a  moderate 
precision  is  desired. 

Other  Radiation  Pyrometers  (Thwing,  Foster,  Brown).  —  In 
Thwing's  apparatus  the  reflecting  mirror  is  replaced  by  a  bright 
cone  which  by  multiple  reflection  concentrates  the  radiation  at 
its  apex  on  one  or  more  thermocouples  in  series  with  a  portable 
galvanometer.  This  apparatus  requires  no  focusing,  but  has 
to  be  sighted  along  the  outside  of  the  tube  or  adjusted  in  the 
direction  until  the  reading  of  the  galvanometer  is  a  maximum 


RADIATION   PYROMETER  285 

when  the  area  sighted  on  is  not  large.  Different  sized  apertures 
may  be  used  at  the  open  end  to  give  different  temperature  ranges. 
Foster  has  also  transformed  the  Fery  telescope  into  a  "  fixed- 
focus  pyrometer  "  (Fig.  105),  by  putting  the  thermocouple  D  and 
the  aperture  EF  at  the  conjugate  foci  of  the  gold  mirror  C.  A 
considerable  area  is  required  to  sight  upon.  This  instrument 
has  to  be  pointed  also  by  trial.  In  a  similar  instrument  recently 


Fig.  105.     Fixed-focus  Pyrometer. 
<« 

issued  by  the  Brown  Pyrometer  Company,  the  sighting  of  the 
instrument  is  facilitated  by  the  use  of  a  finder  such  as  used  with 
photographic  cameras. 

The  Fery  pyrometer  of  constant-focus  type  has  been  coupled 
directly  to  a  long  closed-end  tube  by  Whipple,  thus  rendering 
the  instrument's  readings  independent  of  the  nature  of  the  fur- 
nace or  material  of  which  the  temperature  is  sought,  since  the 
closed  end  tube  is  plunged  directly  into  the  hot  region  or  melted 
metal  to  a  sufficient  depth,  and  the  pyrometer  proper  is  always 
focused  on  the  bottom  of  this  tube  and  indicates  its  temperature. 

Mr.  Whipple  has  used  this  instrument  successfully  for  taking 
molten  steel  and  brass  temperatures  in  the  crucible  at  tempera- 
tures for  the  former  as  high  as  1550°  C.  The  material  of  the 
extension  tube  will  depend  on  the  medium  into  which  it  is  thrust. 

All  of  the  total-radiation  pyrometers  can  be  made  self-regis- 
tering by  simply  substituting  for  the  indicating  galvanometer  a 
suitable  recording  instrument.  The  Fery  mirror  telescope  is 
commonly  used  with  the  Cambridge  thread  recorder  (Fig.  152). 

Some  Experimental  Results.  —  To  call  attention  to  the  scope 
of  application  of  the  radiation  pyrometer,  we  may  note  some  of 
the  investigations  carried  out  with  one  or  another  of  its  forms. 


286  HIGH  TEMPERATURES 

We  have  already  cited  the  early  attempt  at  an  estimation  of  the 
sun's  temperature  by  this  method,  and  we  shall  return  to  this 
matter  in  the  chapter  on  standardization,  as  well  as  to  measure- 
ments on  the  temperature  of  the  carbon  arc  with  the  Fery 
pyrometer. 

Using  his  pyrometer,  Thwing  has  measured  the  total  emissivity 
(page  258,  equation  Ba)  of  streams  of  molten  iron  and  copper. 
For  cast  iron  in  the  liquid  stream  at  1300°  to  1400°  C.,  the  inten- 
sity of  radiation  was  found  to  be  0.29  that  of  the  solid  metal  at 
the  same  temperature,  and  for  mild  steel  at  1600°  to  1650°  C., 
the  value  was  0.28.  These  values  appeared  to  hold  to  1800°  C. 
For  copper  Thwing  finds  the  emissivity  0.14  that  of  a  black  body. 
Burgess,  with  a  Fery  mirror  pyrometer,  finds  for  copper  Ex  =0.15, 
and  for  copper  oxide  Ex  —  0.60,  approximately.  The  observa- 
tions of  Burgess  satisfy  the  following  equations: 

For  liquid  copper:  t  =  3.55  F  —  1018; 

For  cuprous  oxide:  t=  1.41  F  —    169; 

when  t  is  the  true  temperature  centigrade,  and  F  the  reading  of 
the  Fery  pyrometer  calibrated  in  terms  of  the  radiation  from  a 
black  body.  It  will  be  seen  that  at  1083°  C.,  the  melting  point 
of  copper,  the  Fery  instrument  reads  some  490°  low  when  sighted 
on  a  clear  surface  of  pure  copper.  A  Fery  pyrometer  sighted  on 
copper  oxide  at  700°  C.  will  read  higher  than  when  sighted  on  an 
oxide-free  surface  of  liquid  copper  at  1100°  C.  A  similar  inter- 
pretation of  Thwing's  results  on  iron  shows  that  at '1520°  C.,  for 
example,  the  melting  point  of  pure  iron,  a  radiation  pyrometer 
sighted  on  the  clear  metal  would  read  about  370°  low,  assuming 
Es  =  0.29  for  pure  iron.  These  illustrations  are  sufficient  to 
show  that,  with  a  total-radiation  pyrometer,  true  temperatures 
are  given  only  under  carefully  specified  conditions. 
•  Conditions  of  Use.  —  It  will  be  noticed  that  among  the  types 
of  apparatus  described  in  this  chapter,  there  has  been  but  one, 
the  thermopile,  in  its  several  forms  of  thermoelectric  telescope, 
which  has  as  yet  been  used  as  a  pyrometer.  We  have  seen,  how- 
ever, that  several  of  the  other  types  of  radiometric  apparatus 


RADIATION   PYROMETER  287 

may  also  readily  be  made  to  serve  the  purpose  of  temperature- 
measuring  instruments,  and  that  some  of  them  may  in  certain 
cases  offer  theoretical  and  practical  advantages  over  other  forms 
of  pyrometer. 

In  many  industrial  operations  the  temperatures  are  so  high 
that  no  substance  usable  as  an  active  part  of  a  pyrometer,  not 
even  platinum,  can  resist  for  long  their  action  or  that  of  the 
chemical  agents  present.  When  it  is  desired,  therefore,  to  have 
apparatus  of  continuous  indications,  or  with  which  readings  may 
be  obtained  without  intervention  of  an  observer,  and  at  the  same 
time  unalterable  by  heat,  it  is  necessary  to  resort  to  radiation 
pyrometers.  That  with  them  temperatures  may  be  read  off  a 
robust  form  of  portable  millivoltmeter,  and  that  such  instruments 
are  readily  made  self-registering,  are  also  matters  of  great  prac- 
tical convenience. 

It  should  be  emphasized,  however,  that  in  general  such  pyrom- 
eters sighted  upon  objects  in  the  open  air  will  read  too  low  in 
temperature,  due  to  the  selective  radiating  properties  of  all 
materials,  although  radiation  pyrometers  may  be  calibrated  to 
give  true  surface  temperatures  sighted  upon  any  substance  whose 
radiating  properties  (page  259)  are  known;  and  in  any  case  a  self- 
consistent  but  arbitrary  scale  is  obtained  so  long  as  the  surface 
sighted  upon  does  not  change  its  emissivity.  Fortunately,  in 
many  industrial  operations,  these  limitations  are  easily  overcome 
as  well  as  others.  Flames  and  furnace  gases,  which  also  affect 
seriously  the  readings  of  such  pyrometers,  may  be  avoided,  to- 
gether with  the  selective  radiation  errors,  by  sighting  on  the 
bottom  of  a  closed-end  tube  inserted  into  the  furnace.  For 
example,  a  tube  of  fire  clay  or  magnesia,  or  other  material  which 
will  stand  the  temperature  and  chemical  actions  present,  passing 
through  the  lining  of  the  furnace,  and  penetrating  into  the  midst 
of  the  latter  for  a  distance  of  0.5  to  i.o  mm.,  closed  at  the  inner 
end  and  open  at  the  outer,  would  give  a  radiating  surface  at  the 
temperature  of  the  furnace  (Fig.  102),  and  would  approach  very 
closely  to  the  ideal  black-body  conditions  under  which  the  radia- 
tion instruments  will  read  correctly  whatever  be  the  nature  of  the 


288 


HIGH  TEMPERATURES 


object  sighted  upon  at  the  bottom  of  such  a  tube.  The  radiation 
laws  in  their  simplest  form  apply  quite  exactly  to  such  a  radiat- 
ing tube,  so  that  if  the  pyrometer  has  been  calibrated  by  sighting 
it  upon  a  black  body,  the  calibration  will  hold  for  sighting  into  this 
tube,  or  when  sighting  through  a  small  aperture  into  any  clear, 
closed  space  at  constant  temperature.  The  closed-end  tube  above 
mentioned  may  evidently  be  used  in  hardening  baths  and  in 
many  other  kinds  of  industrial  installations. 

Care  has  to  be  taken,  in  setting  up  and  using  any  radiation 
pyrometer,  that  one  allows  sufficient  area  to  sight  upon.     This  is 


T  F 

Fig.  106.     Focusing  on  back  of  Furnace. 


F 
Fig.  107.     Focusing  on  front  of  Furnace. 

illustrated  in  Figs.  105,  106,  and  107.  The  opening  of  the  fur- 
nace F  must  be  large  enough  so  that  the  cone  between  M  and  D 
is  not  cut  into  if  the  instrument  T  is  focused  on  D.  A  smaller 
opening  is  allowable  if  T  is  focused  on  some  such  plane  as  A 
(Fig.  106),  when  the  furnace  is  at  a  uniform  temperature. 

Calibration.  —  It  is  a  difficult  matter  to  calibrate  satisfactorily 
a  total-radiation  pyrometer  to  very  high  temperatures,  and  this 
is  particularly  true  of  most  of  the  industrial  types.  Thus  rela- 
tively large  apertures  are  usually  required,  which  necessitates 
correspondingly  large  furnaces,  over  whose  visible  area  a  constant 
temperature  is  to  be  maintained,  and  measured  by  some  auxiliary 
calibrated  instrument  such  as  a  thermoelectric,  optical,  or  stand- 
.ard  radiation  instrument.  With  a  large  enough  diaphragm  to 


RADIATION  PYROMETER  289 

sight  upon  at  the  center  of  a  uniformly  wound  electric  resistance 
furnace,  unless  great  care  is  taken  in  the  adjustments,  there 
may  exist,  for  example,  differences  of  30°  to  50°  C.  between  the 
two  walls  of  a  2  mm.  thick  diaphragm  as  measured  by  thermo- 
couples. 

The  total-radiation  pyrometer  is  extremely  sensitive,  much 
more  so  than  the  optical  instruments  using  a  single  color,  to  the 
lack  of  blackness,  or  to  selective  emission,  in  the  source  sighted 
upon;  and  this,  combined  with  the  large  aperture  requirement, 
complicates  the  problem.  Furthermore,  as  the  readings  of  these 
instruments  are  considerably  influenced  by  the  presence  of  flames, 
furnace  gases,  and  dust,  it  is  highly  desirable  to  have  a  clear  fur- 
nace to  sight  upon.  While  temperatures  to  1400°  or  1500°  C. 
that  are  exactly  measurable  are  obtainable  in  large  electric  re- 
sistance furnaces  wound  with  platinum  on  porcelain,  or  in  pots 
of  metals  with  closed-end  tubes  inserted  in  the  liquid  or  solid 
metal,  no  entirely  satisfactory  method  of  direct  calibration  free 
from  the  above  sources  of  error  to,  say,  2000°  or  even  1700°  C. 
appears  to  have  been  devised.  Suitable  iridium-tube  furnaces 
lined  with  a  glaze  preventing  evaporation  of  the  iridium  inwards 
could  be  constructed  for  use  to  2000°  C.,  but  the  cost  of  large 
enough  furnaces  of  this  type  would  be  excessive  and  their  life 
short.  Carbon-tube  furnaces  lined  with  the  available  forms  of 
magnesia  or  alumina  are  not  satisfactory,  due  mainly  to  the 
porosity  of  the  lining.  This,  perhaps,  may  be  overcome  by 
building  such  furnaces  so  as  to  prevent  diffusion  inwards  by 
regulating  the  gas  pressure  from  the  center  outwards.  The 
presence  of  any  kind  of  window  before  a  total-radiation  instru- 
ment, especially  if  it  is  to  be  used  as  a  standard,  is  inadmissible, 
due  to  the  unknown  effect  of  the  window  absorption  on  the 
pyrometer  readings.  On  the  other  hand,  this  type  of  pyrometer 
cannot  be  mounted  in  vacuo  for  calibration,  as  its  readings  would 
then  differ  from  those  obtained  in  air,  due  to  different  convection, 
radiation,  and  conduction  conditions  about  the  receiver. 

From  the  fact  that  the  usual  forms  of  industrial  instrument  do 
not,  in  general,  obey  Stefan's  law  exactly,  and  often  not  even 


2QO  HIGH   TEMPERATURES 

approximately,  it  is  not  with  overconfidence  that  any  consider- 
able extrapolation  of  their  temperature  scales  may  be  resorted  to. 

If  a  standard  total-radiation  instrument  can  once  be  calibrated 
satisfactorily  in  any  laboratory,  however,  to  very  high  tempera- 
tures, it  will  then  be  an  easy  matter  to  compare  with  it  any  other 
such  pyrometers  by  sighting  on  any  source  of  radiation  whatever 
with  the  two  instruments  simultaneously. 

It  is  perhaps  proper  to  remark  that  these  difficulties  of  realiz- 
ing suitable  calibration  conditions  impose  equal  uncertainties  in 
the  indications  of  such  instruments  as  ordinarily  used  in  practice. 

The  computation  of  a  calibration  is  readily  made  by  graphical 
methods.  The  equation  E  =  aTn,  in  which  E  is  the  E.M.F. 
generated  for  the  absolute  temperature  T  of  a  black  body,  and 
a  and  n  are  constants,  represents  well  enough  for  most  purposes 
the  behavior  of  thermoelectric  instruments  of  the  Fery  type. 
Plotting  log  E  against  log  T  gives  us  a  straight  line,  so  that  only 
two  points  are  absolutely  necessary  for  calibration  if  the  scale 
of  £  is  correct  and  a  strictly  potential  scale  of  E.M.F.'s.  When 
a  diaphragm  is  used  for  getting  the  higher  temperatures,  one 
observation  combined  with  the  two  previous  ones  will  suffice 
theoretically,  as  this  second  straight  line  should  be  parallel  to 
the  first.  In  practice  this  is  very  nearly  but  apparently  not 
quite  the  case,  due  to  extraneous  heating  and  differences  in  be- 
havior with  the  diaphragm  in  place  and  removed,  so  that  there 
is  some  element  of  uncertainty  in  extrapolating  by  the  above 
procedure. 

Pyrometric  telescopes  may  be  most  accurately  calibrated  by 
the  potentiometric  method  (p.  135),  when  their  readings  are 
independent  of  the  resistances  of  thermocouple  circuit  and  gal- 
vanometer. 


CHAPTER  VIII. 
OPTICAL  PYROMETER. 

Principle.  —  Instead  of  using  the  totality  of  the  radiant  energy, 
as  in  the  methods  described  in  the  preceding  chapter,  use  is  made 
of  the  luminous  radiations  only.  This  utilization  may  be  effected 
in  many  different  ways,  which  give  methods  of  unequal  precision 
and  varying  in  facility  of  manipulation. 

Before  beginning  their  study,  it  may  be  well  to  recall  and  illus- 
trate certain  properties  of  monochromatic  radiation. 

Properties  of  Monochromatic  Radiation.  — An  incandescent  body 
emits  radiations  of  different  wave  lengths.  For  a  given  wave 
length  and  a  given  temperature,  the  intensity  of  this  emitted  radi- 
ation is  not  the  same  for  different  bodies:  this  is  expressed  by 
saying  that  they  have  for  this  radiation  different  emissive  powers. 
Similarly,  a  body  which  receives  radiations  of  a  given  wave 
length  absorbs  a  part  of  them  and  sends  back  another  part  by 
diffusion  or  reflection;  a  certain  quantity  may  also  traverse  the 
body.  The  diffusing,  reflecting,  or  transmitting  power  at  a  given 
temperature,  for  a  given  wave  length,  varies  from  one  body  to 
another.  The  emissive  power  and  the  diffusive  power  (in  the  case 
of  an  opaque  and  nonreflecting  body)  vary  always  inversely,  rest- 
ing complementary  to  each  other. 

Substances  of  great  emissive  power,  as  lampblack,  have  a  small 
diffusive  power;  substances  of  small  emissive  power,  as  polished 
silver  and  magnesia,  have  a  very  great  diffusing  or  reflecting 
power. 

If  we  take  as  the  measure  of  the  emissive  power  the  ratio  of 
the  intensity  of  the  radiation  of  the  object  considered  to  that  of 
a  black  body  (page  239)  at  the  same  temperature,  and  as  measure 
of  the  diffusive  power  the  ratio  of  the  intensity  of  the  radiation 
diffused  to  the  incident  radiation,  the  sum  of  these  two  quantities 
is  equal  to  unity  (see  page  243). 

291 


2Q2  HIGH  TEMPERATURES 

The  emissive  power  of  a  body  varies  from  one  radiation  to 
another,  and  consequently  also  its  diffusing  and  transmitting 
powers,  since  these  two  powers  are  complementary  to  each  other. 
It  follows  that  the  relative  proportions  of  the  visible  radiations 
received  or  given  off  by  a  body  are  not  the  same;  so  that  different 
bodies,  at  the  same  temperature,  appear  to  us  to  be  differently 
colored. 

At  the  same  temperature,  the  color  proper  to  a  body,  and  its 
apparent  color  when  it  is  lighted  by  white  light,  are  comple- 
mentary to  each  other.  Yellow  substances,  as  oxide  of  zinc 
heated,  emit  a  greenish-blue  light.  At  temperatures  less  than 
2000°  the  red  radiations  predominate  greatly  and  mask  the  in- 
equalities of  the  radiations  of  other  wave  lengths.  To  render 
easily  visible  the  colorations  of  radiating  bodies,  it  is  necessary 
to  compare  them  with  those  of  a  black  body  under  the  same 
temperature  conditions.  A  hole  pierced  in  the  body,  or  a  crack 
across  the  surface,  gives  a  very  good  term  of  comparison  to  judge 
of  this  coloration. 

The  intensity  of  the  radiations  emitted  by  a  black  body  in- 
creases always  with  the  temperature,  and  the  more  rapidly  as  we 
approach  the  blue  region  of  the  spectrum;  but,  on  the  other  hand, 
the  radiations  from  the  red  end  are  the  first  to  commence  to  have 
an  intensity  appreciable  to  vision,  so  that  the  color  of  bodies 
heated  to  higher  and  higher  temperatures  starts  with  red,  tend- 
ing towards  white,  passing  through  orange  and  yellow.  White 
is,  in  fact,  the  color  proper  to  bodies  extremely  hot,  as  is  the  sun. 

Bodies  not  black  (the  word  "  black  "  always  being  used  in  the 
sense  of  Chapter  VI,  page  239)  have  a  law  of  increase  different 
from  that  for  black  bodies,  because  the  emissive  power  varies 
with  the  temperature.  It  increases  unequally  for  the  various 
radiations,  so  that  the  color  of  bodies,  with  respect  to  the  color 
of  a  black  body,  changes  with  the  temperature. 

The  following  table  gives  for  different  colors  the  ratios  of  the 
values  of  emissive  powers  of  some  bodies  to  that  of  a  black  body 
as  determined  by  Le  Chatelier.  The  red  radiation  was  observed 
through  a  glass  containing  copper,  the  green  by  aid  of  a  chro- 


OPTICAL   PYROMETER  295 

mium  copper  glass,  the  blue  through  an  ammoniacal  solution  of 
cupric  hydrate.  The  substance  covered  the  junction  of  a  ther- 
moelectric couple,  and  was  cut  by  grooves;  and  it  was  the  bright- 
ness of  the  bottom  of  these  grooves  which  was  compared  to  that 
of  the  surface. 

EMISSIVITIES   (LE   CHATELIER). 

Red.  Green.  Blue. 

Magnesia j at  ^          o.,o          0.15          o.» 

v* • I    1%      :£      :£      :l°o 

t 

Oxide  of  chromium.  . 


1 200  i .  oo  i .  oo  i .  oo 


O^rfthor&ua |      —  ;|°          ;£          ;g 

Oxide  of  cerium..  '   I2°°      '8o    I'°°    l'°° 


1700  i.oo  .40  .30 
1200  .50  .50  .70 
1760  .60  .50  .35 

1200  .80  I.OO  I.OO 

1700  .90  .90  .85 

Admixture {        J~  •£  ;£          ..oo 

Values  of  the  emissivities  of  metals  and  other  substances  are 
given  in  Table  X  of  the  Appendix. 

Methods  of  Temperature  Measurement.  —  The  estimation  of 
temperature,  from  measurements  of  luminous  radiations,  may, 
at  least  in  theory,  be  made  directly  in  three  different  ways,  by 
utilizing : 

The  total  intensity  of  the  luminous  radiation; 

The  intensity  of  a  radiation  of  definite  wave  length; 

The  relative  intensity  of  radiations  of  definite  wave  lengths. 

In  the  chapter  (VI)  on  the  laws  of  radiation  we  have  discussed 
the  recent  theoretical  and  experimental  advances  underlying 
these  methods. 

Measurement  of  Total  Luminous  Intensity.  —  The  brightness 
of  substances  increases  very  rapidly  with  the  temperature.  One 
may  with  the  unaided  eye  estimate  comparatively  this  brightness, 
but  this  measurement  is  very  uncertain,  for  lack  of  a  constant 
standard  of  comparison.  The  sensitiveness  of  the  eye  varies,  in 
fact,  with  the  individual,  with  the  light  which  the  eye  received 
immediately  preceding,  and  with  the  attendant  fatigue.  Photo- 


294  HIGH   TEMPERATURES 

metric  processes,  precise  for  comparison  with  a  standard  source, 
cannot  be  employed  on  account  of  the  change  of  hue  with  the 
temperature. 

The  following  method  might  be  tried:  Trace  on  a  white  surface, 
diffusive  or  translucent  marks,  of  definite  intensity  and  dimen- 
sions, and  seek  what  fraction  of  the  light  must  be  employed  to 
render  the  marks  invisible.  The  indications  will  be  still  quite 
variable  and  will  depend  upon  the  degree  of  the  eye's  fatigue. 

Nernst  and  others  have  made  use  of  the  empirical  formula  (A) 
of  page  238, 


(A) 

/2    vr2/  ' 

connecting  the  total  photometric  brightness,  expressed,  for  ex- 
ample, in  Hefner  candles,  of  a  radiating  body  and  its  temperature. 
Rasch  deduces  theoretically  the  more  general  formula 

7=/iea(l~^,  (B) 


in  which  6  is  the  absolute  temperature  for  which  /  =  /i.  If  /i  is 
the  brightness  of  the  Hefner  standard,  we  have  per  mm:2  of  the 
luminous  radiation  from  a  black  body,  a  =  12.94  and  6=  2068 
abs.  For  small  temperature  differences  (B)  reduces  to  (A)  if 
xT—  const.  In  view  of  the  fact  that  several  investigations 
involving  considerable  temperature  differences  have  been  based 
on  the  use  of  (A),  it  should  be  emphasized  that  (A)  does  not  hold 
unless  TI  is  very  nearly  equal  to  T2. 

Using  (A)  and  taking  the  light  from  i  mm!2  from  a  black  body 
=  12.1  Hefners,  when  the  two  are  of  equal  total  photometric 
brightness,  Nernst  finds  for  the  melting  point  of  platinum 
1782°  and  of  iridium  2200°  to  2240°  C.  Rasch,  treating  Nernst's 
data  by  (B)  gets  for  iridium  2287°  C.  The  same  formulae,  it  is 
interesting  to  note,  apply  also  to  monochromatic  light,  which 
amounts  to  saying  that  (A)  and  (B)  used  for  total  light  give  rela- 
tions equivalent  quantitatively,  as  shown  by  Rasch,  to  Wien's 
law  for  X=  0.542,  or  approximately  for  the  wave  length  of  maxi- 
mum sensibility  of  the  eye,  which  is  not  a  strictly  constant 


OPTICAL   PYROMETER  295 

quantity  for  different  individuals  nor  for  the  same  individual  at 
different  times.  See  page  255  and  Fig.  88. 

This  method  of  using  the  total  photometric  brightness  as  a 
measure  of  temperature,  therefore,  lacks  sensitiveness  as  well  as 
definiteness,  and  is  better  replaced  by  methods  based  on  the  use 
of  a  single  wave  length. 

Measurement  of  the  Intensity  of  a  Simple  Radiation.  —  We 
may  estimate  the  temperature  of  a  body  from  the  intensity  of 
one  of  its  radiations,  provided  that  we  know  the  emissive  power 
of  the  body  at  that  temperature  and  the  law  of  variation  of  this 
radiation  determined  in  terms  of  the  gas  thermometer. 

The  emissive  power  sometimes  varies  with  the  temperature, 
and  generally  is  not  known.  It  might  seem  that  this  would  be 
enough  to  reject  this  method  and  similar  methods  by  radiation. 
But  this  is  not  so,  for  the  following  reasons: 

1.  At  temperatures  higher  than  the  fusing  point  of  platinum 
there  is  no  other  pyrometric  method  at  present  applicable. 

2.  A  great  many  substances  have  a  considerable  emissive 
power,  nearly  unity,  and  particularly  some  of  industrial  impor- 
tance, as  iron  and  coal. 

3.  The  variation  of  radiation  with  temperature  is  sufficiently 
marked  so  that  the  errors  committed  in  neglecting  the  emissive 
power  are  small.    Thus  at  1000°  the  red  radiation  emitted  by 
carbon  is  quadrupled  for  an  interval  of  100°;  it  is  doubled  at 
1500°  for  the  same  temperature  interval. 

Then,  except  for  some  bodies  exceptionally  white,  the  emissive 
powers  at  high  temperatures  are  superior  to  0.5.  By  taking 
them  equal  to  0.75,  the  greatest  error  that  will  be  made  for  the 
ordinary  temperatures  comprised  between  1000°  and  1500°  will 
be  from  25°  to  50°. 

Furthermore,  in  cases  where  the  emissive  power  is  unknown, 
an  optical  pyrometer  will  still  give  a  consistent  temperature  scale 
for  a  given  body,  i.e.,  in  terms  of  black-body  temperatures 
(page  242). 

We  shall  now  describe  the  ordinary  types  of  optical  pyrometer 
and  their  calibration  and  then  indicate  some  applications. 


296 


HIGH   TEMPERATURES 


Optical  Pyrometer  of  Le  Chatelier.  —  Ed.  Becquerel  had  pro- 
posed in  1864  to  refer  the  measurement  of  high  temperatures  to 
the  measurement  of  the  intensity  of  red  radiations  emitted  by 
incandescent  bodies;  but  this  method  had  never  been  realized  in 
a  complete  manner,  and  still  less  employed.  Le  Chatelier,  tak- 
ing up  the  question,  devised  an  experimental  arrangement  suit- 
able for  such  measurements,  and  he  determined  an  empirical  law 
of  radiation  of  substances  in  terms  of  the  temperature. 

Photometer.  —  For  these  measurements  a  photometric  appa- 
ratus is  required  which  gives,  not  as  do  the  ordinary  photometers, 

a  measurement  of  the  total  illu- 
mination produced  by  a  source 
(illumination  which  varies  with 
the  dimensions  of  this  source), 
but  the  intrinsic  brightness 
of  each  unit  of  surface.  Use 
may  be  made  of  a  photometer 
based  on  a  principle  due  to 
Cornu. 

The  apparatus  (Figs.  108  and 
109)  consists  essentially  of  a 
telescope  which  carries  a  small 
comparison  lamp  attached 
laterally.  The  image  of  the 
flame  of  this  lamp  is  projected 
on  a  mirror  M  at  45°  placed  at  the  principal  focus  of  the  tele- 
scope. One  adjusts  for  equality  of  intensity  the  images  of  the 
object  that  is  viewed  and  of  the  comparison  flame,  these  images 
being  side  by  side. 

The  telescope  comprises  an  objective  in  front  of  which  is  placed 
a  cat's-eye  diaphragm  which  admits  of  varying  the  effective 
aperture  of  this  objective,  and,  beyond,  a  stand  destined  to  carry 
tinted  absorbing  glasses. 

At  the  focus  of  the  objective  is  a  mirror  inclined  at  45°  which 
reflects  the  image  of  the  lamp  projected  by  an  intermediary  lens. 
An  ocular,  before  which  is  placed  in  a  set  position  a  monochro- 


Fig.  108.    Le  Chatelier  Pyrometer. 


OPTICAL   PYROMETER 


297 


matic  glass,  serves  for  observing  the  images  of  the  flame  and  of 
the  object. 
To  the  lamp  is  fixed  a  rectangular  diaphragm  which  stops  the 


° 


luminous  rays  not  utilized  and  which  carries  a  stand  to  receive 
tinted  absorbing  glasses. 

The  edge  of  the  mirror  at  45°  is  in  the  plane  of  the  image  of 
the  source  studied,  so  that  the  reflected  image  and  the  direct 


298  HIGH  TEMPERATURES 

image  are  side  by  side,  separated  only  by  the  edge  of  the  mirror. 
This  mirror,  according  to  a  method  devised  by  Cornu,  is  made 
of  a  plate  of  black  glass  cut  with  a  diamond,  which  gives  a  very 
sharp  edge. 

In  order  to  vary  the  relative  intensities  of  the  images,  one  thus 
employs  simultaneously  tinted  glasses  placed  before  one  or  the 
other  of  the  two  objectives,  and  the  cat's-eye  mentioned.  A 
screw  allows  of  varying  the  aperture  of  this  cat's-eye,  and  a  suit- 
able scale  S  indicates  the  dimensions  of  this  opening. 

It  is  very  important  that  the  tinted  glasses  have  an  absorbing 
power  as  uniform  as  possible  and  do  not  possess  absorption 
bands.  These  conditions  are  fulfilled  by  certain  smoked  glasses 
of  ancient  make  (CuO,Fe2O3,Mn02) ;  for  the  fabrication  of  these 
glasses  use  is  now  made  of  the  oxides  of  nickel  and  cobalt,  which 
give  absorption  bands. 

To  determine  the  absorbing  power  of  these  glasses,  a  measure- 
ment is  made  with  and  without  them;  the  ratio  of  the  squares 
of  the  aperture  of  the  cat's-eye  gives  the  absorbing  power. 

For  monochromatic  screens  one  may  use: 

1.  Red  copper  glass,  which  lets  pass  X  =  659,*  about.     The 
use  of  red  glass  is  preferable  to  the  others,  as  it  is  more  nearly 
monochromatic  and  because  measurements  at  low  temperatures 
may  be  made  with  it,  the  first  radiations  emitted  being  red. 

2.  Green  glass  (X  =  546,  about).     The  observations  are  then 
easier  than  in  the  red  for  some  eyes,  but  they  can  be  commenced 
only  at  higher  temperatures. 

3.  Ammoniacal  solution  of  copper  oxide   (X  =  460,   about). 
The  use  of  this  last  screen,  which  is  far  from  monochromatic,  is 
without  interest;  the  eye  is  only  slightly  sensitive  to  the  blue 
radiations,  and  these  last  become  somewhat  intense  only  at  high 
temperatures.     Blue  glass  (Schott  and  Genossen,  No.  F  3875)  is 
preferable. 

*  Red  glasses  furnished  by  the  maker  Pellin,  Paris,  have  an  equivalent 
wave  length  of  about  X  =  632.  Glasses  which  are  more  sharply  monochro- 
matic are  furnished  by  Schott  and  Genossen  of  Jena,  which  firm  also  sup- 
plies very  superior  tinted  absorption  glasses;  see  page  335. 


OPTICAL   PYROMETER 


299 


Adjustment  of  the  Apparatus.  —  There  are  in  the  apparatus 
two  parts  which  require  very  careful  adjustment  for  best  re- 
sults, and  these  parts  should  consequently  be  so  made  as  to 
admit  of  the  necessary  manipulation  to  obtain  the  desired 
effect. 

1.  The  luminous  beam  coming  from  the  lamp  and  which  is 
reflected  by  the  mirror,  and  that  which  comes  directly  from  the 
object  viewed,  should  penetrate  into  the  eye  in  their  totality. 
This  condition  is  fulfilled  if  the  images  of  the  two  objectives 
given  by  the  ocular  are  superposed. 

This  is  verified  by  examining  with  a  lens  these  two  images 
which  are  formed  slightly  behind  the  collar  of  the  ocular.  It  is 
evidently  necessary,  in  order  to  see  them,  to  illumine  the  two 
objectives,  one  with  the  lamp,  the  other  with  any  source  of  light. 
If  the  superposition  does  not  exist,  it  is  established  by  trial  by 
turning  the  screws  which  hold  the  mirror.  If 
it  is  not  too  severely  jarred,  the  apparatus 
should  remain  indefinitely  in  adjustment. 

2.  In  order  that  a  steady  light  may  be  had, 
certain  precautions  in  the  adjustment  of  the 
comparison  lamp  are  necessary.     Le  Chatelier 
recommends  the  employ  of  the  same  gasoline. 
The  flame  should  have  a  constant  height,  equal, 
for  example,  to  the  window  of  the  rectangular 
diaphragm  placed  before  the  flame.    Its  image 
should  be  cut  exactly  in  two  by  the  edge  of  the 
mirror,  a  result  obtained  by  turning  the  lamp 
in  its  stand,  which  is  eccentric  (Fig.  no). 

Finally,  before  taking  an  observation,  it  is 
necessary  to  wait  some  ten  minutes  for  the  lamp 
to  come  into  heat  equilibrium;  then  only  does 
the  flame  possess  a  constant  brightness. 

Measurements.  —  In  order  to  take  an  obser- 


II0- 


vation,  a  body  selected  as  standard,  as  the  flame  Mountins  of  LamP- 
of  a  stearine  candle  or  the  flame  of  a  kerosene  lamp,  is  examined; 
we  observe: 


300  HIGH  TEMPERATURES 

1.  n0,  the  number  of  absorbing  glasses;  *' 

2.  d0,  the  aperture  of  the  cat's-eye; 

3.  /o,  the  extension  of  the  objective  for  focusing. 

The  same  process  is  followed  for  the  source  to  be  studied,  and 
the  numbers  n\,  d\,  f\  are  found. 

k  being  the  absorption  coefficient  of  the  tinted  glasses,  we 
have: 


For  the  glasses  mentioned,  the  absorption  coefficients  are: 

k  —  T\,  corresponding  to  X  =  659; 
k  =  y,  corresponding  to  X  =  546; 
k  =  iV>  corresponding  to  X  =  460. 

For  very  small  objects  which  would  have  to  t>e  placed  very 
near,  a  supplementary  objective  is  put  in  front  of  the  telescope; 
the  object  is  placed  in  the  principal  focus  of  this  new  lens,  the 
objective  of  the  apparatus  being  focused  for  parallel  rays. 
The  absorptive  power  of  this  supplementary  lens  is  reckoned 
as  TV  ^ 

Details  of  an  Observation.  —  The  first  operation  to  make  is 
the  determination  of  the  absorption  coefficients  of  the  absorbing 
glasses.  For  that,  an  object  of  suitable  brightness  is  viewed  once 
with  the  tinted  glass  before  the  cat's-eye  and  then  without  this 
glass.  Let  N  be  the  aperture  of  the  cat's-eye  without  tinted 
glass,  and  Nf  the  aperture  with  such  a  glass.  The  coefficient  k 
of  absorption  is 


-(I)' 


The  following  observations  furnish  data  for  the  determina- 
tion of  the  absorbing  powers  of  different  glasses  employed  in  the 
course  of  studies  relative  to  the  radiations  from  incandescent 
mantles. 


OPTICAL   PYROMETER 


ABSORBING  GLASS  PLACED  BEFORE  THE  SOURCE  TO  BE 

STUDIED. 


Temperature. 

Aperture  of  cat's-«ye. 

Red. 

Green. 

Blue. 

1270° 
1270 

(-f-  1  glass)    . 

IQ-5 
5-5 

21.2 

7-9 

35 
II  .1 

(no  glass)    

kr  =  12.5 

kg  =  7  •  2 

£5  =  9.9 

ABSORBING  GLASS  PLACED  BEFORE  THE  STANDARD  LAMP. 


1170°  (  —  i  glass) 

2    Q 

tr  .QC 

IO.2 

1170    (no  glass) 

0    4 

16.1 

31  .5 

kr  =  10.  5 

kg  =7  -3 

*.-,.s 

Emissive  Power.  —  Before  being  able  to  establish  the  relation 
which  exists  between  the  intensity  of  radiation  of  incandescent 
bodies  and  their  temperature,  it  is  necessary  to  know  the  emissive 
powers  of  these  bodies  (see  page  293).  For  this  measurement  use 
was  made  by  Le  Chatelier  of  the  principle  stated  above,  —  that 
the  interior  of  fissures  in  bodies  may  be  considered  as  inclosed 
in  an  envelope  at  uniform  temperature.  The  emissive  power  is 
thus,  at  the  temperature  considered,  equal  to  the  ratio  of  the 
luminous  intensity  of  the  surface  to  that  of  the  bottom  of  deep 
fissures,  with  the  condition,  evidently,  that  the  aperture  of  the 
fissures  be  sufficiently  small. 

The  body  to  be  studied  was  placed  in  the  state  of  a  paste,  as 
dry  as  possible,  on  the  end  of  a  couple  previously  flattened  so  as 
to  take  the  form  of  a  disk  of  2  or  3  mm.  diameter.  The  drying 
was  very  slow,  so  as  not  to  have  any  swelling  of  the  mass,  and  one 
obtained  in  this  way  a  coating  possessing  fissures;  the  conditions 
described  above  are  then  satisfied.  The  end  of  the  couple  thus 
prepared  is  heated  either  in  a  Bunsen  flame  or  a  blast  lamp,  and 
the  temperature  of  the  junction  is  noted,  while,  simultaneously, 
readings  are  taken  with  the  optical  pyrometer.  In  order  to 


302  HIGH  TEMPERATURES 

obtain  a  temperature  as  constant  as  possible,  it  is  necessary  to 
guard  against  currents  of  air  and  use  a  flame  of  small  size. 
Here  are  some  results  obtained: 

I.  COUPLE  COVERED  WITH  A  MIXTURE  CONTAINING  99  PARTS 
OF  THORIUM  AND  1  OF  CERIUM. 


Temperatures. 

0^0°  (  —  i  glass). 

Red. 

(I)             (2) 

16  o       ... 

Green.                   Blue. 

(I)              (2)              (I)               (2) 
21  .O       I4.O       23  .O        .... 
II  .O          9.O       12.  0       12.  0- 

4-5       3-2       3-5       3-5 

2.O          2.0          1.9          1.9 

5-0     4-0     

.    15.5 

9.0 
3-0 

2.0 

6.0 

137"? 

7.0 

1525                         

3.2 

1650   (+  i  glass)  

8.3 

II.   MAGNESIA. 

1340°  (—  i  glass).  ..  .................    12.2      4.0  18.5  6.7  19.0  9.0 

1460  (—  i  glass)  ....................  4.9   2.5  8.2  3.1  7.7  4.1 

1540  (—  i  glass)  ....................  2.4   1.3  3.1  1.8  3.2  2.1 

The  numbers  give  the  divisions  of  the  cat's-eye;  those  of  column 
(i)  refer  to  the  surface,  and  those  of  column  (2)  to  the  bottom  of 
the  fissures.  The  indications  (—  i  glass)  and  (+  i  glass)  mean 
that  the  absorbing  glass  is  placed  either  before  the  standard  lamp 
or  before  the  source  studied. 

Measurements  of  Intensity.  —  The  following  table  gives  an  idea 
of  the  order  of  magnitude  of  the  intensities  of  different  lumi- 
nous sources,  the  measurements  of  brightness  being  made  in  the 
red.  Unity  is  the  brightness  of  the  axial  portion  of  stearine- 
candle  flame. 

Carbon  beginning  to  glow  (600°)  .................  o.oooi 

Silver  melting  (960°)  ............................  0.015 

Stearine  candle  ......  ] 

Gas  flame  ...........  >•  .........................  i.o 

Acetate  of  amyl  lamp  J 

Pigeon  lamp,  with  mineral  oil  ....................  i  .  i 

Argand  burner,  with  chimney.  .  '.  .................  1.9 

Auer  burner  .....................................  2.05 

Fe3O  melting  (1350°)  ............................  2  .  25 

Palladium  melting  ..............................  4.8 

Platinum  melting  .  .  .............................  15  .  o 

Incandescent  lamp  ..............................  40 

Crater  of  electric  arc  ...........................    10,000 

Sun  at  midday  ..................................    90,000 

Calibration.  —  Le  Chatelier  made  a  first  graduation  of  his. 
optical  pyrometer  by  measuring  the  brightness  of  iron  oxide 
heated  on  the  junction  of  a  thermoelectric  couple,  and  admitting 
that,  for  the  red,  the  emissive  power  of  this  substance  is  equal 


OPTICAL  PYROMETER  303 

to  unity.  He  found  a  law  of  variation  of  the  intensity  of  the 
red  radiations  as  function  of  the  temperature,  which  is  well 
represented  by  the  formula 

3210 

/  =  io6'7  •  T     T  , 

in  which  unit  intensity  corresponds  to  the  most  brilliant  axial 
region  of  the  flame  of  a  candle.  (T  is  absolute  temperature.) 

This  formula  has  been  shown  by  Rasch  to  be  equivalent  to 
(B),  page  294,  for  red  light,  in  which  a  =  13.02.  It  is  therefore 
a  derivative  from  Wien's  law  (page  251). 

The  table  below  gives,  for  intervals  of  100°,  the  intensities  of 
red  radiations  emitted  by  bodies  of  an  emissive  power  equal  to 
unity.  These  numbers  were  calculated  by  means  of  the  inter- 
polation formula  given  above. 

Intensities.           Temperatures.  Intensities.        Temperatures. 

0.00008 600°  39 ,1800° 

.00073 700  60 1900 

.  0046 800  93 2000 

.  020 900  1800 3000 

.078 1000  9,700 4000 

.24... 1 100  28,000 5000 

.64... 1200  56,000 6000 

1.63 1300  100,000 7000 

3.35 1400  150,000 8000 

6.7 1500  224,000 9000 

12.9 1600  305,000 10,000 

22.4. 1700 

These  results  are  represented  graphically  in  Fig.  in. 

After  having  determined  the  value  of  the  diaphragm  opening 
d0,  which  gives  equality  of  brightness  of  the  standard  candle  with 
that  of  the  comparison  lamp,  and  the  absorbing  power  k  of  the 
tinted  glasses,  one  may,  as  was  said  before,  prepare  a  table  which 
gives  directly  the  temperature  corresponding  to  each  aperture  of 
the  cat's-eye. 

With  an  apparatus  for  which 

do  =  5.2,     k  =  iV, 

the  following  table  is  obtained,  in  which  the  plus  sign  refers  to 
tinted  glasses  placed  before  the  objective,  and  the  minus  sign  to 
those  before  the  comparison  lamp. 
This  graduation  applies  to  all  bodies  placed  in  an  inclosure  at 


3°4 


HIGH  TEMPERATURES 


5 

4 
3 
2 
^  1 

1. 

-1 
-2 
-3 

^-' 

^ 

*'''' 

,S 

4 

/' 

. 

/ 

/ 

/ 

/ 

' 

/ 

/ 

/ 

"2.9      3       3.1      3.2      3.3      3.4-     3.5      3.6      3.7      3.8      3.9       4 

Log.  (2+273) 
Fig.  in.    Intensities  in  Terms  of  Temperatures. 

the  same  temperature,  —  in  the  interior  of  furnaces  for  example, 
— and  to  black  bodies  whatever  the  temperature  surrounding 
them;  for  example,  it  applies  very  closely  for  a  piece  of  red-hot 
iron  exposed  to  the  free  air.  For  bodies  whose  emissive  power 
is  inferior  to  unity,  as  platinum,  magnesia,  lime,  it  is  necessary, 
when  they  are  exposed  to  the  air  and  not  surrounded  by  an  in- 
closure  at  the  same  temperature,  to  make  a  special  calibration. 

TYPICAL  CALIBRATION   TABLE   FOR  A   LE   CHATELIER 
OPTICAL  PYROMETER. 


Temperatures. 

—2  glasses,    —i  glass.          o  glass. 

+i  glass. 

+2  glasses. 

7oo°  

17-3             

800  

6.Q             23.0 

QOO  

II.  O               .... 

IOOO    

5.6           18.6 

IIOO    , 

10.5 

1  200    

6.5 

1300    

4-0 

13-6 

1400    

9.4 

1500    

6.6 

I6OO    

4-8 

1  700    

3-6 

12.  O 

I800    

9.1 

IQOO    

7-3 

2000    

5-9 

OPTICAL   PYROMETER  305 

Le  Chatelier  and  Boudouard  made  a  series  of  measurements 
on  radiations  of  different  wave  lengths.  The  junction  of  a  ther- 
moelectric couple  was  placed  in  a  small  platinum  tube,  to  realize 
approximately  an  inclosed  space.  By  taking  as  unity  the  bright- 
ness of  melting  platinum,  the  results  obtained  are  the  following 
for  the  red,  green,  and  blue  radiations: 

TEMPERATURE  VS.  BRIGHTNESS  (IN  TERMS  OF  MELTING 

PLATINUM). 

/         Log(*  +  2?3)  Ir  Log/r  I,  Log/,  Ib  Log/6 


900° 

3.0707 

o  .  0009 

4-95 

0.00018 

4-25 

0.00002 

5-3 

1180 

3.161 

.0024 

3.88 

.0087 

3-94 

.0015 

3-17 

1275 

3.190 

-075 

2.78 

•037 

2-57 

.013 

2.  II 

1430 

3-230 

•23 

1-36 

.16 

1.67 

.058 

2.76 

1565 

3-265 

-72 

1.86 

•47 

i  .20 

•  24 

I".  38 

1715 

3-300 

i  .69 

0.23 

i-45 

o.  16 

•9 

0.95 

Evaluation  of  Temperatures.  —  Finally,  Le  Chatelier  has  used 
his  optical  pyrometer  to  determine  the  very  highest  temperatures 
realized  in  some  of  the  most  important  phenomena  in  nature 
and  in  the  industries.  These  results,  quite  different  from  pre- 
vious determinations,  were  at  first  regarded  with  considerable 
reserve;  they  are  admitted  to-day  as  reasonable,  at  least  within 
the  limits  of  precision,  and  in  terms  of  the  temperature  scale  used 
by  him.  Here  are  some  of  the  figures  obtained: 

Siemens-Martin  furnace 1490°  to  1580°  C. 

Furnace  of  glass  works 1375    to  1400 

Furnace  for  hard  porcelain J37o 

Furnace  for  new  porcelain 1250 

Incandescent  lamp 1800 

Arc  lamp 4100 

Sun 7600 

This  determination  of  the  temperature  of  the  sun,  generally 
believed  to  be  low  at  the  time  it  was  found,  was  confirmed  by 
the  experiments  of  Wilson  and  Gray  (page  271)  by  a  totally  dif- 
ferent method.  Later  determinations  of  the  sun's  temperature, 
using  the  more  recently  established  laws  of  radiation  (Chapter 
VI),  give  values  between  5500°  and  6500°. 

A  series  of  measurements  were  made  with  the  same  apparatus 
in  ironworks.  Here  are  some  results: 


306  HIGH   TEMPERATURES 

BLAST  FURNACE  SMELTING  GRAY  PIG. 

Opening  before  the  tuyere i93°°  C. 

Tapping  the  pig  iron,  beginning 1400 

Tapping  the  pig  iron,  end iS2° 

BESSEMER  CONVERTER. 

Pouring  the  slag 1580° 

Pouring  the  steel  into  the  ladle 1640 

Pouring  the  steel  into  the  molds 1580 

Reheating  of  the  ingot 1200 

End  of  the  hammering 1080 

SIEMENS-MARTIN   FURNACE. 

Flow  of  the  steel  into  the  ladle,  beginning 1580° 

Flow  of  the  steel  into  the  ladle,  end 1420 

Flow  into  the  molds 149° 

Calibration  in  Terms  of  Wien's  Law.  —  As  approximately  mono- 
chromatic radiation  is  used,  the  Le  Chatelier  optical  pyrometer 
may  be  calibrated  in  terms  of  Wien's  law  (III)  (page  251)  by 
sighting  upon  a  black  body  (Fig.  86)  whose  temperature  is  given 
by  means  of  a  thermocouple.  For  this  purpose  Wien's  law  may 
be  written: 

kg /  -  £1  ^- £kL 

where  /  is  the  intensity  of  light,  in  terms  of  the  center  of  the 
Hefner  flame  for  example,  and  T  is  the  absolute  temperature. 
This  method  of  graduation  has  the  advantage  that  only  two 
points  are  required  to  completely  calibrate  the  instrument,  for 

the  relation  between  log  7  and  —  is  linear,  so  that  these  quantities 

being  plotted  give  a  straight  line  which  may  evidently  be  ex- 
tended to  lower  and  higher  temperatures,  since  Wien's  law  has 
been  shown  (page  250)  to  hold  over  the  widest  temperature 
interval  measurable,  provided  the  light  used  is  monochromatic 
and  the  bodies  observed  approximate  blackness  and  are  not 
luminescent,  that  is,  their  light  not  produced  by  chemical  or 
electrical  excitation.  The  value  of  7  is  given  by  the  equation  of 
page  300,  and  for  a  given  absorption  glass  and  focus  is  propor- 
tional to  J2. 

Precision  and  Sources  of  Error.  —  We  shall  give  in  some  detail 
a  discussion  of  the  factors  which  in  the  use  of  the  Le  Chatelier 


OPTICAL   PYROMETER  307 

optical  pyrometer  may  influence  the  photometric  settings  and 
so  affect  the  accuracy  of  temperature  determinations,  as  results 
of  such  a  discussion  are  illustrative  of  what  may  be  expected 
from  optical  pyrometers  in  general.  The  results  are  taken 
from  those  of  Waidner  and  Burgess,  who  have  made  an  experi- 
mental comparison  of  all  the  available  optical  pyrometers. 

The  sources  of  error  of  this  instrument  may  be  those  due  to 
the  standard  Hefner  amyl-acetate  or  other  standard  of  constant 
photometric  intensity  or  temperature  placed  before  the  cat's-eye 
when  adjusting  the  pyrometer,  the  oil  comparison  lamp,  the 
focusing  system,  the  nature  of  the  red  glass  used,  and  the  co- 
efficients of  absorption  of  the  tinted  glasses.  The  first  of  these 
affects  only  comparative  results  with  different  instruments, 
while  the  others,  if  they  exist,  may  be  of  considerable  importance 
in  work  with  a  single  instrument.  We  shall  consider  them  in 
the  order  named. 

As  only  the  central  portion  of  the  amyl-acetate  flame  is  used, 
variations  in  height  and  fluctuations  in  total  intensity  due  to 
various  causes  such  as  moisture  and  carbonic  acid  in  the  atmos- 
phere and  changes  due  to  differing  samples  of  acetate  become 
almost,  if  not  quite,  insignificant  in  this  method  of  comparison; 
so  that,  when  using  only  a  small  central  area  of  the  amyl-acetate 
flame,  it  is  a  very  perfectly  reproducible  standard  under  the 
most  varying  conditions  of  burning.  Again,  the  effects  of  any 
slight  fluctuations  in  light  intensity  are  further  greatly  reduced 
when  transformed  into  temperature  changes,  as  has  been  shown 
(page  238).  Thus,  the  effect  of  varying  the  height  of  the  Hefner 
flame  by  i  mm.,  which  amounts  to  10  per  cent  of  the  total  in- 
tensity when  the  whole  flame  is  used,  causes  a  change  of  less 
than  i  per  cent  in  the  intensity  of  light  from  the  central  area, 
which  is  equivalent  to  less  than  0.5°  C.  change  in  temperature  at 
1000°  C. 

Although  used  intermittently  as  above  indicated,  the  Hefner 
serves  well  enough  as  an  ultimate  standard  by  means  of  which 
the  indications  of  all  photometer  pyrometers  may  be  reduced 
to  a  common  basis,  yet  the  Hefner  is  not  suited  for  use  as 


HIGH  TEMPERATURES 

comparison  lamp  in  the  pyrometer  itself,  as  has  been  previously 
stated. 

In  a  study  of  the  constancy  of  the  comparison  lamp  the  fol- 
lowing arrangement  was  adopted:  In  order  to  obtain  a  perfectly 
constant  source  of  light  with  which  to  compare  the  flame,  a 
32-c.p.  incandescent  electric  lamp  was  placed  in  a  fixed  position 
before  the  objective  of  the  pyrometer  and  a  glass  diffusing  screen 
inserted  before  the  objective.  The  voltage  across  the  lamp 
terminals  was  kept  rigorously  constant,  thus  giving  an  arbitrary 
but  invariable  standard  of  illumination. 

The  concordance  of  results  obtained  by  different  observers  set- 
ting the  gasoline  flame  and  observing  is  shown  below: 

WITHOUT  ABSORPTION  GLASS. 


Cat's-eye  scale  readings  

7-4 
7-4 
7.2 

7-8 
7-9 

7-7 

7-6 

7-8 
7.6 

7-3 
7.0 
8.0 

7-8 
7-7 
7-8 

7.8 

7-7 
7-7 

7-7 
7-8 
7-4 

7-1 
8-3 
8.0 

Means 7.55     7.73     7.65     7.60 

Observers  Nos.  2  and  4  had  no  previous  experience  in  the  use  of 
the  instrument. 

WITH  ABSORPTION   GLASS. 
Observer i  3 


Cat's-eye  scale  readings. 


25.7  25.8 

24.0  24.8 
23.6  26.0 

24.1  25.8 
25.4  24.8 

24.8  24.9 
24.8  25.3 


Means 24.63  25.34 

Here  the  greatest  variation  corresponds  to  less  than  3  degrees  in 
temperature  at  1000°  C. 

To  control  accurately  the  flame  height  in  the  gasoline  lamp,  a 
sight  was  inserted  consisting  of  a  horizontal  scratch  2  mm.  above 
the  window  before  the  flame,  and  a  very  fine  platinum  wire  in 
the  same  horizontal  plane  but  in  a  collar  behind  the  flame.  With 


OPTICAL    PYROMETER  309 

this  improvement  an  observer  can  set  and  control  the  flame 
height  to  0.2  mm.  Such  provision,  however,  is  not  necessary 
except  in  the  most  refined  work,  for  experiment  showed  that  for 
most  purposes  changes  of  over  2  mm.  may  be  made  in  the  flame 
height  with  unimportant  changes  resulting  in  the  temperature 
estimation. 

Considering  the  time  effect  of  burning  upon  the  flame  height 
and  intensity  due  to  local  heating  and  change  of  depth  of  oil,  it 
was  found  that  the  flame  ceases  creeping  up  after  ten  minutes 
and  will  then  remain  at  constant  height  to  within  0.5  mm.  until 
the  oil  is  used  up,  in  three  hours;  and  during  all  this  period  the 
brightness  of  the  flame  does  not  change  by  an  amount  correspond- 
ing to  more  than  5  degrees  in  temperature. 

It  might  be  expected  that  oils  of  different  grades  would  give 
widely  differing  results,  but  an  examination  of  this  possible  source 
of  error  showed  that  different  samples  of  gasoline  and  gasolines 
mixed  with  several  per  cent  of  a  heavy  kerosene  gave  identical 
results.  This  is  of  great  importance  in  the  practical  use  of  the 
instrument,  as  it  shows  that  a  calibration  made  with  a  given 
sample  of  gasoline  remains  good  for  any  other  gasoline. 

From  the  above  it  is  clear  that  variations  in  brightness  of  the 
comparison  flame  due  to  all  possible  causes  need  not  produce 
errors  in  temperature  measurement  of  over  5°  C.  at  1000°  C., 
that  is,  within  the  experimental  limits  of  making  the  photometric 
setting. 

Considering  now  the  sources  of  error  due  to  focusing  and 
sighting  upon  the  object  whose  temperature  is  sought,  it  is  first 
to  be  noticed  that  there  is  a  minimum  distance  from  the  object 
at  which  the  pyrometer  can  be  focused,  this  distance  being 
somewhat  over  a  meter,  depending,  of  course,  upon  the  focal 
length  of  the  objective  and  length  of  drawtube.  There  is  also 
a  minimum  area  which  can  be  sighted  upon  and  give  an  image  of 
sufficient  size  to  completely  cover  the  desired  photometric  field; 
this  minimum  size  of  object  is  about  6  mm.  on  a  side  when  the 
instrument  is  at  its  least  distance;  for  greater  distances  a  larger 
area  must  be  viewed. 


310  HIGH  TEMPERATURES 

The  drawtube  can  easily  be  set  to  2  mm.  when  focusing,  and 
as  the  image  is  over  20  cm.  from  the  objective  in  all  cases,  the 
resulting  error  in  intensity  due  to  focusing  is  not  greater  than 
2  per  cent.  This  corresponds  to  i°  C.  in  temperature,  showing 
that  an  error  of  even  5.  mm.  in  focusing  the  drawtube  will  not 
produce  an  appreciable  error  in  temperature  estimation. 

Often,  in  use,  the  distance  of  the  instrument  from  the  objects 
studied  needs  to  be  changed  considerably,  and  in  rapid  work  it 
is  not  always  convenient  to  ref ocus ;  a  change  in  this  distance  of  a 
fourth  of  its  value,  i.e.,  from  120  cm.  to  150  cm.,  will  produce  an 
apparent  change  in  intensity  of  only  9  per  cent,  or  about  5°  C. 
in  temperature.  That  these  errors  of  focusing  are  so  small 
when  interpreted  into  temperatures,  showing  that  no  unusual 
precautions  are  needed,  is  evidently  of  great  convenience  in  the 
use  of  the  instrument. 

The  nonmonochromatism  of  the  red  glass  in  the  eyepiece 
produces  no  considerable  error  in  temperature  measurement  up 
to  1600°  C.,  although  if  this  glass  is  not  very  nearly  monochro- 
matic the  differences  in  hue  in  the  two  adjacent  photometric 
fields  —  from  the  comparison  lamp  and  other  sources  —  are  very 
troublesome,  and  the  strain  on  the  eye  in  matching  them  is  con- 
siderable. For  the  best  work  at  high  temperatures  a  better  glass 
than  is  usually  furnished  with  the  instrument  must  be  used  (see 
page  335)- 

There  remains  to  be  considered  the  error  introduced  due  to 
uncertainty  in  the  knowledge  of  the.  coefficient  of  absorption  of 
the  absorbing  glasses.  If  an  observation  (Nf)  is  taken  with, 
and  then  one  (N)  without,  an  absorption  glass,  we  have 

ATA2 
k  =  'N^ 


so  that  the  accuracy  in  determining  k  depends  directly  upon  the 
precision  of  setting  and  reading  the  cat's-eye  opening.  Errors 
of  over  5°  at  1000°  C.  can  hardly  occur  from  this  cause,  although 
the  determination  of  k  is  the  most  difficult  and  uncertain  of  all 
the  operations  in  optical  pyrometry. 


OPTICAL   PYROMETER  311 

Modifications  of  the  Le  Chatelier  Pyrometer.  —  For  use  in 

technical  works  and  other  places  where  there  are  sure  to  be  strong 
drafts  of  air  causing  unsteadiness  of  the  flame  of  the  oil  compari- 
son lamp,  the  Le  Chatelier  pyrometer  might  be  improved  by 
the  substitution  of  an  electric  incandescent  lamp  of  low  voltage 
(six)  placed  before  a  uniformly  ground  diffusing  glass  screen, 
which,  illuminated  by  the  incandescent  lamp,  becomes  the  con- 
stant-comparison source.  The  electric  lamp  may  be  mounted 
in  a  vertical  arm  which  serves  at  the  same  time  as  a  handle,  and 
then  the  instrument  becomes  nearly  as  portable  as 'an  opera  glass. 
The  reliability  of  such  a  method  of  producing  a  comparison  light 
of  invariable  intensity  will  be  discussed  when  describing  the 
Wanner  instrument.  Other  modifications  will  be  discussed  under 
the  Fery  and  Wanner  optical  pyrometers. 

The  Shore  Pyro scope.  —  In  this  instrument  (Figs.  112, 112  A)  the 
principle  used  is  similar  to  that  of  the  Le  Chatelier  optical 
pyrometer,  the  parts  being  arranged  in  a  slightly  different  man- 
ner. The  temperature  is  read  directly  off  a  scale  controlled  by 
the  diaphragm;  the  telescope  is  movable  about  a  horizontal  axis, 
and  the  lenses  are  protected  by  easily  removable  cover  glasses. 
In  taking  an  observation,  the  diaphragm  is  turned  until  the 
object  sighted  upon  and  the  flame,  viewed  by  reflection,  are  of 
the  same  brightness. 

Fery  Absorption  Pyrometer.  — This  is  similar  to  Le  Chatelier's 
instrument,  except  that  a  pair  of  absorbing-glass  wedges  p,  p' 
replaces  the  iris  diaphragm;  and  the  45°  mirror  G,  with  parallel 
faces,  is  silvered  over  a  narrow  vertical  strip,  giving  a  photometric 
field  of  form  shown  at  ab,  when  looking  at  a  hot  crucible.  The 
instrument  also  has  a  fixed  angular  aperture,  so  that  no  correction 
has  to  be  made  for  focusing  or  for  varying  distance  from  furnace. 
The  comparison  light  L  plays  the  same  role  as  in  Le  Chatelier's 
pyrometer,  and  the  range  of  the  instrument  may  be  similarly 
extended  by  the  use  of  auxiliary  absorbing  glasses  A,  A1 .  Fery 
has  in  addition  made  his  instrument  movable  about  a  horizontal 
axis,  which  is  a  convenience  in  sighting. 

The  calibration  is  equally  simple.     If  x  is  the  thickness  of  the 


3I2 


HIGH   TEMPERATURES 


Fig.  112.     Shore  Pyroscope. 


wedges,  read  off  on  a  scale,  when  the  light  from  the  comparison 
lamp  and  furnace  is  of  the  same  brightness,  then  the  relation 
between  brightness  I  and  thickness  of  wedge  is 


where  k  is  the  coefficient  of  absorption  of  the  glass  of  the  wedges 
for  the  red  light  used  and  c  is  a  constant. 


OPTICAL   PYROMETER 


313 


Kerosene  Lamp 

R  =  Comparison  Reflector 


Fig.  112  A.     Shore  Pyroscope,  Section. 


314 


HIGH   TEMPERATURES 


A        '    V 


Fig.  113.     Fery  Absorption  Pyrometer. 

But  by  Wien's  law  III  (page  251),  assuming  it  to  apply  here, 


or  combining  these  two  equations  we  have 

B 


whence 


Cekx  =  Ae    T, 
kx  +  C '  = 


Thus  it  follows  that  the  thickness  of  the  wedge  is  inversely  pro- 
portional to  the  absolute  temperature,  so  that  the  calibration 


OPTICAL   PYROMETER  315 

may  be  effected  by  finding  the  thickness  of  wedge  for  two  tem- 
peratures only  and  plotting  a  straight  line  and  constructing  a 
table  giving  7  and  T  respectively  in  terms  of  x. 

It  is  questionable  if  there  is  any  gain  in  substituting  the  wedge 
for  the  cat's-eye  in  the  desire  to  extend  the  range  over  which  the 
instrument  may  be  used  without  employing  the  auxiliary  absorb- 
ing glasses,  for  thereby  the  sensibility  is  somewhat  reduced,  and, 
more  important  still,  the  wedge  instrument  cannot  be  used  at 
such  low  temperatures  as  the  original  Le  Chatelier  form,  nor  is 
there  any  gain  in  simplicity  of  calibration  and  ease  of  manipula- 
tion. The  shape  of  the  photometric  field,  the  use  of  an  aperture 
of  constant  angle,  and  making  the  instrument  movable  about  a 
horizontal  axis,  however,  are  improvements  over  the  Le  Chate- 
lier instrument;  and  the  Fery  instrument  enjoys  the  further 
advantages  that  it  may  more  conveniently  be  sighted  on  small 
objects,  and  fewer  absorption  glasses  are  needed. 

Wanner  Pyrometer.  —  Wanner,  making  use  of  the  polarizing 
principle  discarded  by  Le  Chatelier,  has  brought  out  a  photom- 
eter pyrometer  which  is  a  modification,  suited  to  temperature 
measurements,  of  Konig's  spectrophotometer. 

The  comparison  light  is  a  6-volt  incandescent  lamp,  illuminat- 
ing a  glass  matt  surface;  monochromatic  red  light  is  produced 
by  means  of  a  direct- vision  spectroscope  and  screen  cutting  out 
all  but  a  narrow  band  in  the  red,  and  the  photometric  comparison 
is  made  by  adjusting  to  equal  brightness  both  halves  of  the 
photometric  field  by  means  of  a  polarizing  arrangement. 

The  slit  Si  is  illuminated  by  light  from  the  comparison  source, 
a  small  4- volt  electric  lamp  (Fig.  115),  not  shown  in  the  Fig.  114, 
reaching  Si  after  diffuse  reflection  from  a  right-angled  prism 
placed  before  Si.  Light  from  the  object  whose  temperature  is 
sought  enters  the  slit  $2.  The  two  beams  are  rendered  parallel 
by  the  lens  LI,  and  each  dispersed  into  a  continuous  spectrum 
by  the  direct-vision  prism  P.  Each  of  these  beams  is  next 
separated  by  a  Rochon  prism  R  into  two  beams,  polarized  in 
planes  at  right  angles.  Considering  only  the  red  light,  there 
would  now  be  four  images  formed  by  the  lens  L%  and  distributed 


HIGH  TEMPERATURES 


« 


about  the  slit  54.  In  order  to  bring  two  red  images  oppositely 
polarized  exactly  before  this  slit,  a  bi-prism  B  is  interposed  whose 
angle  is  such  as  to  effect  this  for  two  images 
only,  at  the  same  time  increasing  the  number 
of  images  to  eight.  There  is  now  in  the  field  of 
view  before  the  Nicol  analyzer,  A,  two  contigu- 
ous red  fields  composed  of  light  oppositely 
polarized,  the  light  of  one  coming  from  Si  alone, 
and  of  the  other  from  S%  alone.  All  the  other 
images  are  cut  off  from  the  slit  S^.  If  the 
analyzer  is  at  an  angle  of  45°  with  the  plane  of 
polarization  of  each  beam,  and  if  the  illumina- 
tion of  5*1  and  52  is  of  the  same  brightness,  the 
eye  will  see  a  single  red  circular  field  of  uniform 
brightness.  If  one  slit  receives  more  light  than 
the  other,  one-half  of  the  field  will  brighten,  and 
the  two  may  be  brought  to  equality  again  by 
turning  the  analyzer  carrying  a  graduated 
scale,  which  may  be  calibrated  in  terms  of 
temperature. 

If  the  analyzer  is  turned  through  an  angle  0 
to  bring  the  two  halves  of  the  field  to  the  same 
p  brightness,  the  relation  between  the  two  inten- 

. 

sities  from  Si  and  Sz  is 


1  =  tan2  0. 


Calibration.  —  Since  monochromatic  light  is 
used,  and  the  comparison  beam  and  that  from 
the  object  examined  undergo  the  same  optical 
changes,  Wien's  law  III  may  form  the  basis  of 
the  calibration. 

As  constructed  and  generally  used,   the  45° 
position  of  the  analyzer  when  setting  on  the 
standard  corresponds  most  conveniently  to  some  intermediate 
arbitrarily  chosen  position  on  the  graduated  scale  (Fig.   115) 


OPTICAL   PYROMETER 


317 


of  the  instrument.  This  reference  position  or  "normal  point" 
is  the  scale  reading  to  which  the  instrument  must  be  adjusted, 
by  varying  the  current  through  the  comparison  lamp  or  its 
distance  from  the  slit  Si,  when  sighting  upon  the  standard  amyl- 
acetate  flame.  The  positions  of  the  flame  and  pyrometer  are 
fixed  mechanically  (see  Fig.  115).  The  flame  height  must  be 
carefully  adjusted  and  the  lamp  should  burn  some  ten  minutes 
before  standardizing. 


Fig.  115.     Wanner  Pyrometer. 


If  70  is  the  intensity  of  light  from  the  standard  amyl-acetate 
lamp,  TQ  the  corresponding  equivalent  temperature  absolute, 
and  $o  the  reading  in  degrees  on  the  scale  of  the  instrument  for 
the  "  normal  point,"  and  /,  T,  and  <f>  are  the  intensity,  apparent 
temperature,  and  scale  readings  when  sighting  upon  the  object 
whose  temperature  is  sought,  we  have 


tan 


tan2  <f> 


(a) 


assuming  the  circle  to  be  uniformly  graduated  and  the  optical 
parts  in  adjustment.     Also  Wien's  law  III  (page  251)  gives  us 


318  HIGH  TEMPERATURES 

Since  the  constant  c2  =  14,500  for  a  black  body  and  X  =  0.656  /* 
as  the  instrument  is  usually  constructed,  a  knowledge  of  the 
apparent  black-body  temperature  of  the  standard  source,  to- 
gether with  the  reading  of  the  analyzer  scale  at  the  normal  point 
when  /  =  /o,  for  such  an  instrument,  is  all  the  data  required  for 
its  calibration,  as  any  temperature  may  then  be  calculated  by 
means  of  equations  (a)  and  (b)  in  terms  of  the  scale  readings. 
The  apparent  temperature  T0  of  the  amyl  acetate  may  be  taken 
as  1673°  aDS-  or  1400°  C.  This  instrument  may  also,  of  course, 
be  empirically  calibrated  in  terms  of  the  readings  of  a  thermo- 
couple, using  a  black  body  to  sight  upon  (see  p.  241). 

The  actual  computation  involved  in  a  calibration  is  very 
simple,  and  is  readily  done  graphically  in  a  manner  similar  to 
that  suggested  for  the  Le  Chatelier  optical  pyrometer.  From 
equations  (a)  and  (b)  above  we  have 

log  tan  </>  =  a  +  ft.,  ......     (c) 


so  that  if  log  tan  </>  is  plotted  in  terms  of  —  ,  we  have  a  straight 

line  of  which  b  is  the  tangent  and  a  the  intercept  on  the  log  tan  <£ 
axis.  If  a  and  b  are  known  for  the  type  of  pyrometer  used,  a 
single  calibration  temperature  suffices,  otherwise  two  observa- 
tions of  T  and  0  are  required  to  completely  solve  (c)  .  It  is  safer, 
however,  to  take  several  temperatures  and  draw  the  line  best 
representing  the  observations  according  to  (c).  A  table  or  a 
curve  of  <f>vst  (  =  7^  —  273)  may  then  be  constructed  for  practi- 
cal use. 

It  is  evidently  necessary  to  be  able  to  always  reproduce  exactly 
the  standard  intensity  70.  Now,  the  brightness  of  an  electric 
lamp  will  vary  with  the  current  through  it,  so  it  is  necessary  to 
check  frequently  the  constancy  of  illumination  of  the  slit  Si 
against  a  standard  source  of  light.  The  amyl-acetate  lamp  and 
ground-glass  diffusing  screen  are  placed  before  the  slit  S2,  thus 
reproducing  the  standard  light  required.  The  analyzer  is  then 
set  at  the  previously  determined  normal  point  and  the  distance 
of  the  electric  lamp  from  Si  adjusted  or  the  current  through  the 


OPTICAL   PYROMETER  319 

lamp  changed  by  a  rheostat,  until  the  two  fields  appear  of  the 
same  brightness. 

In  the  latest  form  of  this  instrument  the  details  of  its 
mechanical  construction  have  been  improved,  and  it  has  been 
made  direct-reading  by  providing  a  second  scale  on  the  instru- 
ment graduated  in  temperatures,  corresponding,  of  course,  to 
a  definite  normal  point  and  for  a  source  approximating  a  black 
body. 

The  amyl-acetate  standardizing  lamp  may  be  eliminated 
wholly  or  in  part  in  the  use  and  calibration  of  the  Wanner  pyrom- 
eter. If  the  electric  comparison  lamp  be  fixed  in  position,  the 
reading  of  the  instrument  sighted  on  a  black  body  at  a  single 
temperature,  as  the  gold  point  (adjusting  the  scale  to  a  conven- 
iently located  "normal  point"),  or  better,  at  a  series  of  known 
temperatures,  may  be  taken,  for  a  definite  current  through  the 
comparison  lamp.  If  this  same  current  is  always  maintained  in 
the  use  of  the  instrument,  this  calibration  will  hold  as  long  as 
the  lamp  does  not  change  nor  the  optical  parts  of  instrument 
become  deranged.  This  last  method  of  use  is  preferable  in  exact 
work  where  calibrating  apparatus  is  available.  The  normal 
point,  however,  may  still  be  that  given  by  the  amyl  acetate  if  so 
desired.  Or,  the  amyl-acetate  standard  with  its  corresponding 
normal  point  may  be  retained,  but  used  only  occasionally  for 
checking  and  adjusting  the  constancy  of  the  pyrometer,  whose 
uniformity  of  indications  is  maintained  in  the  meantime  by 
keeping  the  current  constant  in  the  comparison  lamp  when  tak- 
ing measurements,  and  keeping  the  comparison  lamp  in  a  fixed 
position. 

According  to  Nernst  and  Wartenberg,  it  may  also  be  necessary 
to  correct  the  circle  readings  by  a  constant  fraction;  thus  they 

found  that  for  a  certain  Wanner  instrument  the  ratio — 

tan2  <fe' 

taken  over  the  scale  by  means  of  a  sector  disk,  was  not  constant, 
but  that  the  expression  anz  n>  -  was  constant,  where  m  is  nearly 
unity. 


320  HIGH  TEMPERATURES 

Sources  of  Error.  —  A  study  of  a  Wanner  instrument  by  Waid- 
ner  and  Burgess  has  led  them  to  the  following  conclusions.  The 
sensibility  of  this  pyrometer  varies  with  change  in  the  angle, 
and  is  so  adjusted  as  to  be  the  greatest  between  1000°  and  1500°  C.,, 
and  is  about  as  follows: 

o.i  scale  div.  o  i°  C.  at  1000°  C. 
o.i  scale  div.  =c=  2°  C.  at  1500°  C. 
o.i  scale  div.  ^  7°  C.  at  1800°  C. 

The  reproducibility  of  the  brightness  of  the  amyl-acetate  flame 
as  viewed  through  the  ground-glass  diffusing  screen  is  a  measure 
of  the  ability  of  the  instrument  to  repeat  its  indications.  It  is 
very  important  that  this  diffusing  screen  be  always  placed  in 
exactly  the  same  position  relative  to  the  flame  and  slit  S^  and 
further,  that  it  be  free  from  dust  and  finger  marks.  These  re- 
quirements can  only  be  satisfactorily  met  by  protecting  this 
screen  by  a  cover  glass  and  providing  an  adjustment  for  setting 
it  exactly  in  place  between  the  flame  and  slit. 

The  constancy  of  the  amyl-acetate  flame  as  used  with  this 
pyrometer  under  ordinary  conditions  of  burning  is  illustrated 
by  the  following  set  of  observations,  during  which  the  current 
through  the  electric  comparison  lamp  was  kept  rigorously  con- 
stant by  means  of  a  milliammeter  and  rheostat: 


Reading  of  instrument. 

Deviations. 

39-9 

—  0.28 

39-9 

-0.28 

40.1 

-0.48 

39-9 

—  0.28 

+0.52 

39-2 

+0.42 

39-8 

-O.l8 

39-0 

+0.62 

39.6  0.38 

This  shows  that  the  flame  can  be  relied  upon  to  give  an  intensity 
of  illumination  whose  constancy  expressed  in  terms  of  tempera- 
ture is  0.5  per  cent.  Variations  in  height  of  the  flame,  if  they  do 
not  exceed  2  to  3  mm.,  together  with  fluctuations  in  atmospheric 
conditions,  will  not  produce  errors  in  temperature  estimation  ex- 
ceeding i  per  cent. 


OPTICAL   PYROMETER 


321 


The  uncertainty  of  setting  the  Nicol,  due  to  lack  of  sensitive- 
ness of  the  eye  to  exactly  match  the  two  halves  of  the  photometric 
field,  is  also  about  i  per  cent,  or  slightly  better  with  practice. 

The  adjustment  of  the  electric  lamp  to  standard  intensity  at 
the  point  on  the  scale  chosen  as  normal  point  can  be  made,  when 
proper  care  is  taken  regarding  the  diffusing  screen,  to  i  per  cent 
expressed  in  temperature  change.  This  source  of  error  does  not 
affect  relative  results  in  any  one  series  for  one  setting  to  the  normal 
point. 

The  most  serious  source  of  error,  except  when  special  pre- 
cautions are  taken,  is  the  variation  in  brightness  of  the  electric 
comparison  lamp  due  to  variation  in  the  current  furnished  by 
the  three-cell  storage  battery. 

With  the  lo-ampere-hour  battery  furnished  with  the  Wanner 
instrument,  after  making  circuit  the  electromotive  force  drops 
by  about  2  per  cent  in  two  minutes  and  then  falls  off  slowly,  but 
nearly  recovers  the  original  voltage  after  remaining  on  open 
circuit  even  for  a  very  short  time.  When  the  battery  is  in  good 
condition  the  variation  in  three  hours  at  normal  discharge  (0.75 
ampere)  is  about  0.08  volt,  and  somewhat  less  for  the  current 
(0.55  ampere)  taken  by  the  lamp;  with  the  battery  in  poor  con- 
dition these  changes  are  much  accentuated. 

The  following  table  illustrates  the  effect  of  slight  variations 
in  current  through  the  lamp  on  apparent  temperature  of  the 
amyl-acetate  flame,  for  the  small  battery  of  10  ampere  hours 
furnished  with  the  instrument.  The  apparent  change  in  tem- 
perature is  calculated  from  the  current  change: 

SMALL  BATTERY. 


Time. 
Minutes. 

Wanner  scale. 

Current  in 
amperes. 

Per  cent  change 
in  current. 

Apparent  change 
in  temperature. 

I  r 

31  .2 

0.5645 

2O 

31-8 

0.5640 

O.I 

i°C. 

27 

32.7 

0-5550 

i-7 

10 

37 

34-6 

0.5400 

4-3 

25 

36 

Disconnected  battery  two  minutes. 

40 

32.5 

0.5610 

0.6 

3 

42 

3J-7 

0.5570 

!-5 

7 

45 

32.5 

0.5560 

2-5 

15 

47 

33-i 

0.5505 

4-i 

24 

322  HIGH  TEMPERATURES 

A  battery  of  75  ampere  hours  gave  similar  results. 

The  above  results  give  abundant  evidence  of  the  need  of 
maintaining  the  current  through  the  lamp  quite  constant  in 
work  of  precision.  A  series  of  experiments  has  shown  that  in 
the  range  1000°  to  1500°  C.  one  division  on  the  Wanner  scale 
corresponds  to  about  0.009  ampere,  or  i°  C.  apparent  change 
in  temperature  is  produced  by  a  fluctuation  of  0.0012  ampere 
through  the  lamp;  hence  to  obtain  a  precision  of  5°  the  current 
must  be  kept  constant  to  o.oi  of  its  value.  The  above  table 
shows  that  this  is  by  no  means  effected  by  using  the  battery 
without  regulating  the  current,  for  even  with  the  battery  in  the 
best  condition  the  current  increases  by  2  per  cent  in  the  first 
eight  or  nine  minutes  of  discharge  and  then  falls  off  i  per  cent  in 
the  next  twenty  minutes.  The  temperature  coefficient  of  the 
battery  would  produce  only  insignificant  changes.  The  table 
shows  further  that  breaking  the  circuit  and  then  making  it  again 
may  cause  an  apparent  temperature  change  of  over  20°  C.  For 
work  of  precision,  therefore,  it  is  essential  to  keep  the  current 
constant  by  means  of  a  milliammeter  and  rheostat,  otherwise  un- 
certainties of  over  25°  C.  will  occur  in  the  temperature  measure- 
ments. These  will  increase  with  the  battery  in  poor  condition. 

Range  and  Limitations.  —  The  above  description  of  the  Wan- 
ner pyrometer  has  shown  the  great  loss  of  light  due  to  the  optical 
system  employed.  This  prevents  measuring  temperatures  below 
about  900°  C.  (1650°  F.)  with  this  instrument.  There  is  no 
method  of  sighting  this  pyrometer  exactly  upon  the  spot  desired, 
except  by  trial,  as  no  image  of  the  object  examined  is  formed  in 
the  eyepiece,  but  this  inconvenience  is  in  part  compensated  by 
not  having  to  focus  with  varying  distance  from  the  object. 

There  is  another  limitation  which  may  in  certain  cases  become 
a  serious  source  of  error:  light  from  incandescent  surfaces  is,  in 
general,  polarized,  and,  as  the  Wanner  instrument  is  a  polarizing 
pyrometer,  care  must  be  taken  to  eliminate  this  source  of  error 
when  it  exists. 

If  an  incandescent  object  is  viewed  normally,  the  amount  of 
polarized  light  is  very  small,  but,  as  the  angle  of  incidence  in- 


OPTICAL   PYROMETER  323 

creases,  the  proportion  of  light  polarized  becomes  greater  and 
greater.  Besides  varying  with  the  angle  of  incidence,  the  amount 
of  polarized  light  emitted  varies  widely  with  different  substances, 
being  greatest  for  polished  platinum  and  very  much  less  for 
iron,  glass,  etc.  In  some  measurements  made  with  the  Wanner 
pyrometer  on  the  temperature  of  an  incandescent  platinum  strip 
in  the  neighborhood  of  1350°  C.,  Waidner  and  Burgess  have  found 
a  maximum  difference  in  the  readings  of  op°  C.  for  positions  of 
the  instrument  at  right  angles  to  one  another  in  azimuth  and  for 
an  angle  of  incidence  of  70°  with  the  normal  to  the  surface. 
This  introduces,  under  these  conditions,  the  possibility  of  an 
error  of  45°  C.  in  the  temperature  measurement.  This  source 
of  error  can  be  eliminated  by  taking  the  mean  of  four  readings 
for  azimuths  90°  apart.  The  magnitude  of  the  error  arising 
from  this  cause  is  entirely  negligible  for  all  practical  purposes 
for  many  substances,  such  as  iron,  porcelain,  etc.  A  considerable 
area  is  needed  to  sight  upon  with  this  pyrometer,  which  is  a  dis- 
advantage when  small  objects  are  viewed  from  a  distance. 

Due  to  the  relatively  large  surface  required  in  sighting  the 
Wanner  pyrometer,  there  is  a  tendency  to  bring  the  instrument 
too  close  to  the  furnace  or  object  viewed,  and  this  practice  carried 
to  excess  may  readily  damage  the  instrument,  deranging  its 
optical  parts  and  altering  the  calibration  by  very  considerable 
amounts.  Warning  of  overheating  is  sometimes  given  by  the 
change  in  color  of  the  field.  Placing  a  water  jacket  between  the 
furnace  and  instrument  or  otherwise  screening  the  latter  will 
evidently  obviate  this  difficulty. 

Where  an  attempt  is  made  to  sight  on  very  small  or  distant 
areas,  such  as  wires  or  narrow  strips  which  fill  only  a  small  part 
of  the  photometric  field,  there  may  be  produced  diffraction  effects, 
as  noticed  by  Hartmann. 

A  review  of  the  sources  of  error  and  limitations  of  the  Wanner 
pyrometer  shows  that  they  may  exert  a  relatively  great  effect  on 
the  temperature  measurements,  and  it  was,  therefore,  thought 
worth  while  to  emphasize  them;  but,  on  the  other  hand,  they  may 
all  be  practically  eliminated  with  reasonable  care,  and  the  instru- 


324 


HIGH  TEMPERATURES 


ment  then  becomes  one  of  great  precision  and  convenience,  for 
those  measurements  for  which  it  is  adapted.  We  shall  see  later 
how  its  range  may  be  extended  to  the  highest  temperatures. 

Instrument  for  Low  Temperatures.  —  In  order  to  render  his 
pyrometer  available  for  temperatures  below  900°,  Wanner  has 
brought  out  a  modification  suitable  for  use  from  625°  to  1000°, 
with  two  ranges,  625°  to  800°  and  800°  to  1000°  C.,  which  gives 
a  very  open  scale  and  renders  the  instrument  available  for  a  great 
many  industrial  operations  that  were  hitherto  inaccessible  to  it. 
In  this  low-temperature  form,  shown  in  Fig.  116,  the  light  from 


Fig.  116.     Wanner  Outfit  for  Low  Temperatures. 

the  furnace  does  not  pass  through  the  polarizing  system,  and  the 
direct-vision  prism  is  replaced  by  a  red  glass  in  the  eyepiece,  by 
which  elimination  light  of  much  feebler  intensity  than  with  the 
high-range  apparatus  can  be  observed.  The  apparatus  is  very 
compact  and  easy  to  manipulate.  It  requires  as  accessories  a 
4- volt  storage  battery,  milliammeter,  and  amyl-acetate  standard. 
Holborn-Kurlbaum  and  Morse  Pyrometers.  —  If  a  sufficient 
current  is  sent  through  the  filament  of  an  electric  lamp,  the  fila- 
ment glows  red  at  first,  and  as  the  current  is  increased  the  fila- 
ment, getting  hotter  and  hotter,  becomes  orange,  yellow,  and 
white,  just  as  any  progressively  heated  body.  If  now  this 
filament  is  interposed  between  the  eye  and  an  incandescent 


OPTICAL   PYROMETER 


325 


object,  the  current  through  the  lamp  may  be  adjusted  until  a 
portion  of  the  filament  is  of  the  same  color  and  brightness  as  the 
object.  When  this  occurs  this  part  of  the  filament  becomes 
invisible  against  the  bright  background,  and  the  current  then 
becomes  a  measure  of  the  temperature  as  given  either  by  a  ther- 
mocouple or  in  terms  of  the  intensity  of  illumination.  This 
principle  appears  to  have  been  first  used  by  Morse  and  inde- 
pendently developed  by  Holborn  and  Kurlbaum.  An  absolute 
match  of  both  color  and  brightness  cannot  be  made  unless  mono- 
chromatic light  is  used  or  unless  the  lamp  filament  and  viewed 
object  radiate  similarly. 


45'Mirror 
Absorbing  Screen 


Secfion  on  A-C 

Fig.  117.     Holborn-Kurlbaum  Pyrometer. 

Holborn-Kurlbaum  Form.  —  A  small  4-volt  electric  incandes- 
cent lamp  L  with  a  horseshoe  filament  is  mounted  in  the  focal 
plane  of  the  objective  and  of  the  eyepiece  of  a  telescope  provided 
with  suitable  stops  D,  D,  Z),  and  a  focusing  screw  S  for  the 
objective.  The  lamp  circuit  is  completed  through  a  two-cell 
storage  battery  B,  a  rheostat,  and  a  milliammeter. 

The  determination  of  a  temperature  consists  in  focusing  the 
instrument  upon  the  incandescent  object,  thus  bringing  its 
image  into  the  plane  AC,  and  adjusting  the  current  by  means  of 
the  rheostat  until  the  tip  of  the  lamp  filament  disappears  against 
the  bright  background,  when  a  previous  calibration  of  current, 
in  terms  of  temperature  for  the  particular  lamp  used,  gives  the 
temperature  by  reading  the  milliammeter. 


326  HIGH  TEMPERATURES 

As  the  temperature  of  the  filament  increases,  the  effect  of  irra- 
diation or  too  great  brightness  becomes  blinding,  and  the  photo- 
metric comparison  is  then  rendered  possible  at  these  temperatures 
by  the  introduction  of  one  or  more  monochromatic  red  glasses 
before  the  eyepiece,  giving  as  well  all  the  advantages  of  photom- 
etry of  a  single  color.  Below  800°  C.  the  measurements  are 
more  easily  made  without  any  red  glass,  as  the  filament  itself  is 
then  red,  and  the  lowest  temperatures  are,  of  course,  reached  with 
the  least  interposition  possible  of  absorbing  media.  The  lower 
limit  of  the  instrument  is  very  nearly  600°  C.  Two  red  glasses 
are  required  for  temperatures  above  1200°  C.,  and  for  very  high 
temperatures,  above  1500°  or  1600°  C.,  it  is  necessary,  in  order 
to  avoid  overheating  the  lamp  filament  by  the  current,  to  put 
absorbing  glasses  or  a  double-prism  mirror  (Fig.  121)  before  the 
objective;  and  they  also,  of  course,  require  calibration.  At  very 
high  temperatures,  unless  a  strictly  monochromatic  glass  is  used, 
the  pyrometry  becomes  difficult,  the  filament  never  disappearing 
completely. 

The  eye  is  particularly  sensitive  in  recognizing  equality  of 
brightness  of  two  surfaces,  one  in  front  of  the  other,  and  this 
pyrometer,  therefore,  provides  a  very  delicate  means  of  judging 
temperatures,  since  the  light  intensity,  as  has  been  shown  (page 
238),  varies  so  much  faster  than  does  the  temperature. 

The  precision  attainable  with  this  pyrometer  is  illustrated  by 
the  following  series  of  observations  which  are  indicative  of  the 
ordinary  performance  of  the  instrument: 


Temperature  from 
H.-K.  pyrometer. 

Temperature  from 
thermocouple. 

Temperature  from 
H.-K.  pyrometer. 

Temperature  from 
thermocouple. 

1347 

i347°C. 

632 

634°  C. 

I3SI 

1347 

634 

633 

1343 

1343 

633 

633 

1333 

1332 

633 

632 

1342 

1342 

Different  observers  do  not  differ  by  any  appreciable  amount 
in  their  readings,  and  at  low  temperatures  the  same  values  are 
obtained  whether  a  red  glass  is  used  or  not. 


OPTICAL  PYROMETER  327 

For  the  calibration  of  the  instrument,  it  is  necessary  to  find 
empirically  the  relation  between  the  current  through  the  lamp 
and  the  temperatures  for  a  number  of  temperatures,  and  then 
interpolate  either  analytically,  or  more  conveniently,  graphically. 
The  calibration  will  evidently  be  an  independent  one  for  each 
lamp  used. 

The  relation  between  current  and  temperature  is  sufficiently 
well  expressed  by  a  quadratic  formula  of  the  form 

C  =  a  +  bt  +  ct2. 

That  this  formula  gives  satisfactory  results  is  shown  by  obser- 
vations of  Holborn  and  Kurlbaum  for  a  lamp  satisfying  the 
equation 

C  io3  =  170.0  +  0.1600 1  +  0,0001333  t2, 

when  sighted  on  a  black  body  (page  239),  the  temperature  of 
which  is  given  by  a  thermoelectric  pyrometer  calibrated  at  known 
melting  points. 

C  amp.  io- 3.  t  obs.  t  calc.  A*. 

340  686  679  -7°C. 

375  778  778  o 

402  844  850  +6 

477  1026  1032  -f-6 

552  1196  1196  o 

631  1354  1354  o 

712  1504  1504  o 

We  may  also  cite  the  behavior  of  one  of  the  several  standard 
pyrometer  lamps  of  the  Bureau  of  Standards.  This  lamp  satis- 
fies the  equation 

C  =  o.i 68 1  +  0.03  1482  t  +  o.o6  i7oo/2. 


C  in  amps. 

fobs. 

/calc. 

At 

o  .  4486 

920 

921 

-1° 

.5305 

1087  .  5 

1087.5 

O 

•3357 

650 

649 

+  1 

.6023 

1221 

1221 

0 

.3525 

692 

692.5 

-0-5 

•6393 

1285 

1285 

o 

•5309 

1089 

1088.5 

+0-5 

No  appreciable  change  in  the  readings  of  this  lamp  could  be 
detected  over  a  five-year  period,  the  lamp  being  used  very  fre- 
quently during  that  time  to  temperatures  as  high  as  1500°  C. 


328  HIGH  TEMPERATURES 

Pirani  and  Meyer  have  shown  that,  for  carbon  and  metal 

filament  lamps, 

log  C  =  a  +  b  log  T 

where  C  =  current  and  T  =  absolute  temperature.     This  per- 
mits of  a  calibration  with  two  temperatures  only. 

Mendenhall  suggests  that  this  pyrometer  —  and  the  same  is 
true  of  all  the  optical  instruments  using  monochromatic  light  — 
may  be  calibrated  for  all  temperatures  in  terms  of  a  single  known 
temperature,  such  as  the  palladium  melting  point,  by  means  of 
a  series  of  sectored  disks  each  of  a  different  aperture,  giving,  by 
the  application  of  Wien's  law  (see  page  250),  a  corresponding 
series  of  effective  temperatures.  The  sectors,  of  some  15  cm.  di- 
ameter, may  be  rotated  by  means  of  a  shaft  attached  to  a  small 
motor  fixed  near  the  middle  of  the  outside  of  the  pyrometer 
tube.  Mendenhall  has  also  made  a  direct-vision  spectroscopic 
eyepiece  for  this  instrument,  and  works  with  a  field  of  about 
25  A.U.  width,  giving  X  to  about  one-fifth  per  cent  in  the  middle 
of  the  visible  spectrum. 

Holborn  and  Kurlbaum  as  well  as  Waidner  and  Burgess  have 
made  a  thorough  study  of  the  effects  of  aging. 

Lamps  which  have  not  been  aged  or  burned  for  some  time  at 
a  temperature  considerably  above  that  at  which  they  will  ordi- 
narily be  used,  undergo  marked  changes  and  are  unreliable,  but, 
if  properly  aged,  they  reach  a  steady  condition,  as  indicated  by 
the  following  table  of  results  obtained  by  Holborn  and  Kurlbaum 
on  these  lamps.  The  current  is  given  in  each  case  for  a  tempera- 
ture of  uoo°C. 

AGING  OF   LAMPS. 

Current. 


Lamp  number I                    2  3 

After  20  hours  burning  at  1900°  C ». . . .     0.608  0.592  o  589 

After    5  hours  burning  at  1900°  C 613           .592  .592 

After    5  hours  burning  at  1900°  C .621  .597  .597 

After    5  hours  burning  at  1900°  C 622  . 599  .600 

After  20  hours  burning  at  1500°  C 622  . 599  .601 

If  a  lamp  is  not  aged  its  indications  may  change  by  as  much 
as  25°  C.  with  time,  but  after  twenty  hours'  heating  at  1800°  it 
will  undergo  no  appreciable  further  changes  over  a  period  of 


OPTICAL   PYROMETER  329 

time  corresponding  to  many  months  if  used  in  the  shop,  if  not 
heated  above  1500°.  This  state  of  permanence  is  sufficient  to 
satisfy  the  most  rigid  requirements  of  practice. 

By  the  substitution  of  tungsten  for  carbon  filaments  even 
greater  permanence  may  be  had,  but  the  selective  radiation  of 
the  metallic  filament  may  then  be  a  source  of  error  or  inconven- 
ience in  certain  cases. 

Morse  Thermogage.  —  In  its  original  form,  instead  of  a  simple 
horseshoe  filament,  Morse  used  a  large  spiral  filament  in  the 
lamp  of  his  pyrometer,  so  that  in  sighting  upon  an  incandescent 
body  it  was  necessary  to  choose  some  particular  spot  of  the  spiral 
and  try  to  make  that  spot  disappear.  This  is  fatiguing,  as  the 
spiral  covers  a  large  area  and  is  of  just  sufficiently  varying  inten- 
sity to  cause  the  eye  to  wander.  This  effect  was  aggravated  by 
the  fact  that  this  instrument  was  not  a  telescope,  possessing  no 
eyepiece  or  objective,  so  that  the  eye  had  to  accommodate  itself 
back  and  forth  between  the  filament  and  the  object  studied. 

Instead  of  the  4-volt  battery  for  the  Holborn-Kurlbaum 
lamps,  the  spiral  lamp  took  a  battery  of  40  or  50  volts,  requir- 
ing a  costly  installation  unless  the  fluctuations  of  the  ordinary 
no- volt  lighting  circuit  were  not  too  troublesome  to  use  it  with 
a  suitable  rheostat  or  shunt. 

The  Morse  instrument  was  designed  for  use  in  hardening  steel, 
and,  throughout  the  limited  temperature  range  required  in  this 
process,  in  spite  of  the  crudities  of  construction  above  noted,  this 
pyrometer  could  be  read  to  about  3°  C.  within  this  range.  Above 
iioo°C.,  however,  it  is  very  difficult,  and  it  soon  becomes 
impossible  to  make  a  satisfactory  setting. 

Tests  of  these  spiral  filament  lamps  show  that  when  aged  at 
1200°  C.  they  will  remain  constant  for  several  hundreds  of  hours 
within  the  range  over  which  they  are  intended  to  be  used. 

It  is  interesting  in  this  connection  to  note  the  behavior  of 
ordinary  carbon  incandescent  lamps  as  to  permanence.  (See 
Fig.  118.) 

Later  forms  of  the  Morse  thermogage  are  provided  with  lower 
voltage  lamps  with  a  single  loop,  red  glass  at  the  eyepiece,  and 


330 


HIGH  TEMPERATURES 


made  into  a  telescope,  following,  in  part,  suggestions  given  to 
Morse  by  Waidner  and  Burgess. 


rfO 


100 


200     HOURS 


400    425 


500 


Fig.  118.     Behavior  of  Carbon  Lamp. 

Henning's  Spectral  Pyrometer.  —  In  order  to  eliminate  the 
uncertainties  and  corrections  for  the  lack  of  monochromatism 
of  colored  glasses  used  with  the  Holborn-Kurlbaum  instrument, 
and  to  permit  temperature  measurements  with  any  colored 
light,  Henning  has  devised  a  spectral  pyrometer  suitable  for 


© 


Fig.  119.     Henning's  Spectral  Pyrometer. 

exact  work  in  the  laboratory  from  1000°  C.  It  is  essentially  a 
combination  of  the  Holborn-Kurlbaum  instrument  with  a  spec- 
trometer as  shown  in  Fig.  119.  The  collimator  KLz,  telescope 
FLi,  carrying  an  observing  slit  D  or  an  ocular,  and  Abbe  prism  P 
which  can  be  set  to  give  any  wave  length  by  means  of  the  mi- 
crometer M NA,  together  with  the  slit  E  adjustable  in  width  by 


OPTICAL  PYROMETER  331 

the  screw  U,  constitute  the  spectrometer.  An  image  of  the  in- 
candescent body  is  superposed  on  the  lamp  G  by  the  lens  Z,4,  and 
both  are  seen  in  colored  light  with  the  observer's  eye  before  D. 
The  screen  B  carries  a  series  of  suitable  stops.  The  micrometer 
scale  A  is  calibrated  in  wave  lengths  by  means  of  light  from 
standard  sources,  as  helium  and  mercury  vacuum  tubes.  The 
instrument  may  also  be  arranged  for  use  as  a  spectrophotometer. 
Henning  has  used  his  spectral  pyrometer  in  a  study  of  metal- 
filament  lamps  and  for  the  determination  of  absorption  and 
reflecting  coefficients  of  metals.  He  has  shown  that,  for  a 

series  of  metals,  the  equation  —  —  —  =  const.,  in  which  5  and  So 

o      oo 

are  the  absolute  black-body  temperatures  for  wave  lengths  X 
and  X0,  holds  over  a  wide  range  of  temperatures;  and  that  the 
absorption  coefficients  remain  practically  constant  with  change 
of  temperature. 

Calibration  of  Optical  Pyrometers.  —  We  have  already  called 
attention  to  the  fact  that  the  most  accurate  method  of  calibrating 
an  optical  pyrometer  to  about  1600°  C.  is  to  take  its  readings 
when  sighted  into  an  experimental  black  body  (page  239)  whose 
temperature  is  best  given  by  two  or  more  thermocouples  which 
have  in  turn  been  calibrated  by  determining  their  E.M.F.'s  at 
the  freezing  points  of  three  or  more  pure  metals.  These  calibra- 
tions are,  in  general,  best  left  to  a  properly  equipped  standard- 
izing laboratory.  However,  it  is  often  desirable  to  be  able  to 
calibrate,  at  least  approximately,  one's  own  optical  pyrometer, 
even  if  not  in  the  possession  of  a  complete  standardizing  equip- 
ment. 

A  fair  substitute  for  the  black  body  is  a  resistance-tube  fur- 
nace of  the  Heraeus  type  with  a  diaphragm,  say  a  piece  of 
graphite,  inserted  at  its  center,  or  a  little  back  of  this,  and  on 
which  the  optical  pyrometer  is  sighted.  The  temperature  of  this 
diaphragm  may  be  obtained  with  a  calibrated  thermocouple  or 
optical  pyrometer.  Sighted  into  such  a  furnace,  whose  total 
length  is  some  twenty  or  thirty  times  its  diameter,  an  optical 
pyrometer  will  read  some  5°  to  15°  C.  too  low. 


332  HIGH  TEMPERATURES 

The  following  method  may  also  be  used,  and  this  requires  no 
auxiliary  pyrometer,  but  does  require  from  one  to  three  or  more 
deep  crucibles  of  substances  of  known  melting  points,  preferably 
the  pure  metals,  such  as  Al  or  Sb,  Cu,  Ni,  or  Fe.  The  optical 
pyrometer  is  sighted  on  the  bottom  of  a  porcelain  tube,  preferably 
blackened  inside,  and  which  is  thrust  into  the  melted  metal, 
and  the  reading  of  the  pyrometer  taken  at  the  freezing  point 
of  the  metal. 

Where  several  optical  pyrometers  are  in  use  in  the  same  estab- 
lishment, it  is  well  to  have  at  least  one  of  them  calibrated  care- 
fully and  kept  as  a  standard.  The  others  are  readily  calibrated 
by  comparing  their  readings  with  that  of  the  standard  when 
sighted  on  any  convenient  incandescent  source  whatever,  pro- 
vided the  pyrometers  all  use  the  same  colored  light;  otherwise  it 
is  safer  to  use  a  furnace  as  source,  although  graphite  or  iron 
(oxide)  will  answer  in  most  cases. 

The  criterium  of  a  satisfactory  comparison  source  for  pyrom- 
eters using  different  colors  is  to  view  the  source,  when  this 
is  possible,  with  different  colored  glasses  applied  in  succession 
to  one  pyrometer.  If  the  same  reading  is  obtained  for  all  — 
red,  yellow,  and  green,  for  example  —  the  source  is  satis- 
factory. 

The  Wide-filament  Comparison  Lamp.  —  A  very  convenient 
and  rapid  method  of  standardizing  one  optical  instrument  in 
terms  of  another  is  shown  in  Fig.  120,  which  was  devised  by 
Waidner  and  Burgess  for  the  determination  of  incandescent 
lamp  filament  temperatures  and  the  melting  points  of  very 
refractory  metals.  Fig.  120  illustrates  the  use  of  a  carbon  strip  C 
mounted  in  vacuo  for  the  former  purpose.  The  standard  pyrom- 
eter L  and  the  lamp  F  whose  filament  temperature  is  sought  are 
both  brought  to  the  same  brightness  as  C,  and  the  currents  in 
L  and  F  give  a  measure  of  their  temperatures,  which  are  assumed 
equal  if  the  color  of  the  glass  G  is  the  same  as  that  used  before  L 
and  if  the  filaments  F  and  L  are  of  the  same  material.  The 
lenses  E  and  0  make  the  readings  of  F  more  convenient  and 
equalize  the  two  optical  systems.  Evidently  any  type  of  optical 


OPTICAL  PYROMETER 


333 


pyrometer  may  be  substituted  for  the  lamp  F  and  calibrated  in 
a  similar  manner. 

These  carbon-strip  comparison  lamps  may  be  used  intermit- 
tently to  temperatures  as  high  as  1800°  C.  or  even  2000°  C.  If 
used  only  at  comparatively  low  temperatures,  they  may  them- 
selves be  calibrated  in  terms  of  current  vs.  temperature  and  then 
serve  as  a  secondary  standard,  replacing  the  black  body.  Such 
lamps  of  this  type  as  are  at  present  available  change  pretty 
rapidly  with  even  short  burning,  so  that  it  is  better  to  keep  a 
filament  lamp  or  other  optical  pyrometer  as  the  standard  and 


Fig.  120.     Calibrating  Method  of  Waidner  and  Burgess. 

use  the  wide  strips  merely  as  comparison  sources.  For  extend- 
ing such  comparisons  to  higher  temperatures,  it  would  be  de- 
sirable to  replace  the  carbon  with  tungsten  strips,  when  probably 
2500°  C.  or  more  could  be  realized. 

Other  comparison  sources  are  available,  however,  for  these  very 
high  temperatures,  such  as  the  Arsem  vacuum  furnace  (Fig.  176) 
with  which  temperatures  of  nearly  3000°  C.  may  be  attained,  and, 
moreover,  black-body  conditions  are  completely  realized. 

Use  of  Wedge-shaped  Cavities.  —  We  have  already  seen  that 
in  the-  calibration  of  his  optical  pyrometer  Le  Chatelier  took 
advantage  of  crevices  in  heated  materials  surrounding  a  thermo- 
couple to  obtain  approximately  black-body  conditions.  Fery 
has  called  attention  to  the  necessity  of  the  measuring  instrument 
also  being  black  in  the  Kirchhoff  sense,  at  least  when  absolute 


334  HIGH  TEMPERATURES 

measurements  are  made,  and  he  developed  the  use  of  conical 
receivers. 

Mendenhall,  studying  the  relation  between  true  and  apparent 
temperatures  of  metals  by  means  of  the  optical  pyrometer,  shows 
that  if  a  thin  metal  strip  is  bent  into  a  wedge  of  small  angle,  the 
radiation  from  within  the  wedge,  heated  electrically,  as  is  a  lamp 
filament,  is  very  nearly  that  of  a  black  body;  so  that  simul- 
taneous readings  with  a  calibrated  pyrometer  on  the  outside  and 
inside  of  such  a  wedge  give  a  measure  of  the  selective  properties 
of  its  substance.  The  wedge  may  also  replace  the  black  body 
for  the  comparison  of  one  optical  pyrometer  with  another. 
Assuming  specular  reflection  and  the  wedge  angle  Z,,  the  number 

of  reflections  perpendicular  to  the  edge  of  the  wedge  is  n  = ; 

J^j 

if  the  reflecting  power  of  its  material  is  r,  that  of  the  wedge  is  rn. 
For  many  metals  r  is  of  the  order  of  0.7  for  red  light,  when  for  a 
lo-degrees  wedge  rn  =  0.0016  and  e  =  ae  =  0.998  e,  corresponding 
to  a  temperature  difference  from  a  black  body  of  the  same  bright- 
ness of  only  0.5°  C.  at  1600°.  For  matt  surfaces  the  departure 
from  blackness  is  greater.  The  difference  in  temperature  be- 
tween the  inner  and  outer  surfaces  of  the  wedge  is  less  than  i°  C. 
for  metals  of  0.04  mm.  or  less  in  thickness.  By  burning  out  such 
wedges  of  platinum,  Mendenhall  and  Faryther  obtained  a  value 
for  the  platinum  melting  point  only  8  degrees  lower  than  the 
figure  of  Waidner  and  Burgess  (1753°  C.). 

Monochromatic  Glasses.  —  In  order  to  use  Wien's  law  with 
exactness  and  convenience,  and  especially  when  extrapolation 
on  the  temperature  scale  is  resorted  to,  it  is  highly  desirable  that 
there  be  no  change  in  the  color  of  the  light  used  in  an  optical 
pyrometer.  With  those  pyrometers  in  which  the  monochro- 
matic light  is  produced  by  means  of  colored  glasses,  there  may  be 
an  error  introduced  due  to  the  lack  of  homogeneity  of  the  light 
transmitted  and  to  the  consequent  shift  with  temperature  in  the 
position  of  maximum  intensity  of  the  light.  For  such  inhomoge- 
neous  glasses  this  is  equivalent  to  introducing  a  continuous  change 
of  wave  length  with  temperature  in  Wien's  law  (page  251). 


OPTICAL   PYROMETER 


335 


The  behavior  of  certain  Jena  glasses,  which  are  among  the 
best  in  the  smallness  of  this  effect,  as  found  by  Waidner  and 
Burgess,  is  shown  in  the  following  table: 

MONOCHROMATISM   OF   COLORED   GLASSES    (JENA). 


Glass. 

Thickness 
in  mm. 

Temper- 
ature of 
source  (C). 

Xinax. 

Limits  of 
transmission  band. 

Red,  No. 

274.C 

•}   04 

(  IOOO 
<  I2?O 

0.645/x 
6^0 

0.698^1-0.  6  1  0/A 
.731     -    .602 

Red,  No. 

274.C 

6  o1? 

f  1450 
I4CO 

.656 
661 

.772     -    .598 

.  7C3      -     .608 

Green,  N 

3.  43  1  m    . 

6  18 

i  H50 

•  547 

.602      ~     .532 

Blue,  No. 

3086  

4-32 

(  145° 

(  1320 
(   1470 

.546 

.462 
.462 

.631      ~     .468 

.500      -     .421 
.511      -     .408 

The  position  of  the  optical  center  of  gravity  (Xmax.  in  the  table) 
is  seen  to  remain  stationary  for  the  green  and  blue,  but  to  shift 
slightly  to  longer  wave  lengths  for  the  red  glass,  with  increase 
in  temperature.  An  error  of  0.005^1  in  the  estimation  of  the 
equivalent  wave  length  for  a  colored  glass  corresponds  to  an 
error  in  temperature  estimation  of  about  5°  C.  at  1750°  C. 

For  some  of  the  newer  monochromatic  Jena  glasses  the  follow- 
ing data  on  the  transmission  coefficients  have  been  issued  by 
Schott  and  Genossen: 

TRANSMISSION  COEFFICIENTS  (£>)  OF  JENA  GLASSES  FOR 
1  MM.  THICKNESS. 


Glass. 


Fraction  transmitted  for  wave  lengths  (in  M). 


Type. 

Name. 

X  =  0.644 

0.578 

0.546 

0.509 

0.480 

0.436 

F  4SI2 

Red  filter 

O   O4. 

o  o^ 

F  2745.  .  .  . 

Copper-ruby  

O.72 

O.  3Q 

0.47 

0.47 

0.45 

0.43 

F43I3-..- 
F435I-... 
F4937-.-. 
F  4930.  .  .  . 

Yellow  glass,  dark  .  .  . 
Yellow  glass,  medium 
Yellow  glass,  light.  .  . 
Green  filter  

0.98 
0.98 
I  .OO 

o  17 

0.97 
0.97 

I.  00 

o  so 

0-93 
0.96 

1.  00 

o  64 

0.83 

0-93 
0.99 
o  62 

0.09 
0.44 
0.74 
o  44 

o.iS 

0.40 

F3875  

Blue  filter  

o  18 

o  so 

0.73 

F38i5.... 

Neutral  black  

o  35* 

0.35* 

o  37* 

0.35* 

* 
0.34 

0.30* 

For  a  thickness  of  o.i  mm. 


336 


HIGH  TEMPERATURES 


The  fractional  transmission  Dx  for  any  other  glass  thickness  xx 
is  given  by  the  expression  Dx  =  Dx,  where  D  is  the  transmission 
for  i  mm.  as  given  in  the  table. 

Extension  of  Scale.  —  All  of  the  optical  pyrometers  based  on 
the  use  of  a  single  wave  length,  such  as  the  Le  Chatelier,  Wanner, 

and  Morse,  may  have  their  scales 
indefinitely  extended  by  the  use  of 
neutral  absorbing  glasses  (such  as 
Jena  Rauch  Glas) ,  reflecting  mir- 
rors, or  prisms  of  black  glass  (see 
Fig.  121),  or  sectored  disks,  placed 
between  the  furnace,  or  other  source 
whose  temperature  is  to  be  meas- 
ured, and  the  pyrometer. 

The  same  principle  for  the  com- 
puting of  temperatures  with  the 
screen  in  place  applies  for  all  of 
these  screens  and  for  any  of  these 
pyrometers.  It  is  only  necessary 


Fig.  121.    Absorption  Mirrors. 


to  find  the  absorption  coefficient  of  the  screen  for  the  colored  light 
used  with  the  pyrometer.  This  absorption  coefficient  may  be 
calculated  by  making  use  of  Wien's  law  (page  251)  and  from 
observations  at  one  or  more  temperatures.  Thus,  if  K  is  the 
absorption  factor,  that  is,  the  reciprocal  of  the  absorption  co- 
efficient, TI  and  Tz  the  apparent  temperatures  in  degrees  abso- 
lute given  by  the  pyrometer,  sighting  on  a  black  body  first 
without  and  then  with  the  absorbing  screen,  then  Wien's  law 
III  gives 

^      ,      /i      c2l( 
logio  A  =  log  —  : 

when  ^2  =  14,500  for  a  black  body,  and  X  is  the  wave  length  in 
M  ( =  o.ooi  mm.)  of  the  light  used  by  the  pyrometer.  Applied 
to  the  high-range  Wanner  and  Henning  spectral  pyrometers,  the 
above  formula  applies  exactly  to  the  highest  attainable  tempera- 
tures if  the  absorbing  screen  has  a  constant  coefficient  for  all 


OPTICAL  PYROMETER 


337 


brightnesses;  but  for  those  pyrometers  using  colored  glasses, 
which  are  never  strictly  monochromatic,  there  will  be  an  error 
entering  into  the  extrapolations,  which  can,  however,  for  the 
most  part,  be  eliminated  by  the  calibration  in  wave  length  vs. 
temperature  of  the  colored  glasses  used,  as  shown  in  the  pre- 
ceding paragraph. 

That  these  corrections  can  be  made  satisfactorily  is  shown 
by  the  following  from  the  data  of  Waidner  and  Burgess  on  the 
determination  of  the  melting  point  of  platinum  by  means  of  a 
Holborn-Kurlbaum  pyrometer  using  red,  green,  and  blue  glasses 
and  provided  with  different  kinds  of  absorbing  screens.  The 
metals  were  melted  in  an  iridium-tube  furnace  approximating 
very  closely  a  black  body.  The  observations  of  Nernst  and 
Wartenberg  with  a  Wanner  pyrometer  using  yellow  light '  are 
also  included,  for  comparison,  their  results  being  reduced  to 
the  same  optical  basis,  i.e.,  for  c<i  =  14,500  in  Wien's  formula. 
Measurements  by  the  same  observers  on  palladium  gave  equally 
concordant  results. 

ELIMINATION  OF  CORRECTIONS  TO  OPTICAL  PYROMETERS. 


Observers. 

Absorbing  screen. 

Absorption 
factor. 

Wave 
length. 

Number 
of  obser- 
vations. 

Melting  point 
of  platinum. 

Waidner  and 
Burgess.  .  .  . 

Reflecting 
mirrors  
Reflecting 
mirrors  
Sector  disk   . 

}iQ9 

228 

•2f    A 

0.668 

0-547 
0.668 

23 

7 
10 

i753°±3  C. 
i75i   ±3 

1753    i2 

Nernst  and 
Warten- 
berg. 

Sector  disk  , 
Sector  disk  

>  Rauch  glass  

35-4 
35-4 

147 

o-547 
0.462 

0.5896 

6 
4 

4 

1748    ±2 

1749  ±3 
1750  ±5 

For  most  materials  heretofore  used  as  absorbing  screens, 
either  of  the  mirror  or  transmitting  glass  type,  there  is  a  rapid 
variation  in  absorbing  factor  with  wave  length  of  the  incident 
light  (see  page  335  and  above  table).  Schott  and  Genossen  of 
Jena  now  furnish  a  "  neutral  black  "  glass  (F  3815)  of  an  absorb- 
ing factor  which  remains  very  constant  throughout  the  visible 


338  HIGH  TEMPERATURES 

spectrum.  The  fractional  transmission  for  this  glass  is  given  in 
the  table  on  page  333. 

The  use  of  a  sector  disk  is  preferable  for  exact  work  in  the 
laboratory  where  the  intensity  of  the  source  observed  has  to 
be  cut  down,  for  this  form  of  screen  has  a  constant  absorption 
factor  which  may  be  determined  geometrically  with  great  exact- 
ness. The  absorbing  glasses  are  usually  more  convenient  to  use 
than  the  reflecting  mirrors  and  are  equally  as  good,  or  better. 

Some  Scientific  Applications.  —  Our  knowledge  of  phenomena 
occurring  at  very  high  temperatures  has  been  increased  greatly  in 
the  past  few  years,  largely  due  to  the  availability  of  convenient 
and  precise  optical  pyrometers  using  monochromatic  light.  We 
shall  pass  briefly  in  review  some  of  the  uses  to  which  this  type 
of  instrument  has  been  put  in  the  laboratory  as  illustrations  of 
what  may  be  accomplished  in  high-temperature  measurements 
by  optical  means. 

Temperature  of  Flames.  —  Any  substance  inserted  in  a  flame 
will  take  up  a  lower  temperature  than  that  of  the  flame  itself, 
due  to  conduction,  radiation,  and  diminished  speed  of  the  gas 
stream  around  the  body.  E.  L.  Nichols,  by  using  thermocouples 
of  progressively  finer  wires,  sought  to  determine  true  flame 
temperatures  by  extrapolating  for  a  wire  of  zero  diameter.  The 
uncertainty  of  this  method  is  considerable,  although  it  gives 
consistent  results,  which  are  probably  low. 

The  radiation  methods  have  been  employed  by  several  experi- 
menters. The  temperature  as  given  by  an  optical  pyrometer 
will  depend  on  the  thickness  and  density  of  the  flame  as  well 
as  upon  its  reflecting  and  absorbing  powers.  The  reflecting 
power  of  a  flame  is  small  and  probably  varies  with  the  kind  of 
flame;  the  results  as  yet  obtained  are  quite  discordant  on  this 
point. 

Kurlbaum  interposed  a  flame  between  a  black  body  and  the 
eye  and  assumed  that  the  two  were  of  the  same  temperature 
when  the  flame  disappeared  against  its  background.  This 
method  gave  results  lower  than  those  obtained  by  Lummer  and 
Pringsheim  (page  252).  Kurlbaum  and  Stewart  both  claim  that 


OPTICAL  PYROMETER  339 

the  carbon  in  the  flame  departs  more  widely  from  a  black  body 
than  platinum,  and  the  latter  gets  2282  for  the  value  of  A 
in  Wien's  displacement  equation  \mT=  A,  assuming  Nichols's 
value  1900°  C.  for  the  acetylene  temperature.  Fery  has  shown, 
however,  that  the  brightness  of  the  sodium  line,  measured  with 
a  spectrophotometer,  is  not  increased  by  passing  obliquely  a 
beam  from  an  electric  light  across  the  flame  studied,  seeming  to 
indicate  that  the  diffusing  power  is  nil  for  the  light  coming  from 
carbon.  This  would  imply  a  value  of  A  of  the  order  of  2800,  or 
of  2400°  C.  for  the  acetylene  flame,  assuming  Xw  =  1.05. 

Fery's  method  of  measuring  flame  temperatures  is  to  produce 
the  reversal  of  a  metallic  line  by  means  of  light  emitted  by  a 
solid  body  brought  to  the  proper  temperature.  The  image  of 
the  filament  of  an  incandescent  lamp  is  thrown  by  a  large-aperture 
lens  onto  the  narrow  slit  of  a  spectroscope.  The  rays  from  the 
filament  pass  through  the  flame  to  be  studied,  which  contains 
sodium  or  other  metallic  vapor.  When  the  filament  is  raised  in 
temperature  the  D  line,  say,  is  ultimately  reversed,  and  at  the 
moment  of  disappearance  the  filament  and  flame  are  assumed  to 
have  the  same  temperature,  which  may  be  measured  by  sighting 
an  optical  pyrometer  on  the  filament. 

Some  of  Fery's  results  are  as  follows: 

(  Open 1870°  C. 

Bunsen  j  Half-open 1810 

(  Shut 1710 

Acetylene 2550 

Oxyhydrogen  with  illuminating  gas  and  oxygen 2200 

Oxyhydrogen  with  H2  +  O 2420 

For  this  determination  Fery  used  his  absorption  pyrometer.  The 
results  obtained  may  be  slightly  high,  but  hardly  by  more  than 
100°  C.,  as  a  fine  wire  of  platinum  may  be  melted  in  an  open 
Bunsen. 

There  have  been  other  estimations  of  apparent  temperatures  of 
flames  by  various  optical  methods  based  on  the  radiation  laws, 
some  of  which  have  given  values  greatly  below  the  true  tempera- 
tures, as  measured  by  the  ability  of  these  flames  to  melt  refrac- 
tory materials  of  known  melting  point. 


34O  HIGH  TEMPERATURES 

Making  use  of  Wien's  displacement  law  in  form  Xmax  T  =  2940, 
Ladenburg  found  1405°  for  the  Hefner  and  1842°  for  the  acety- 
lene flame.  Becker,  by  a  spectrophotometric  method,  obtained 
1395°  for  the  Hefner. 

Kurlbaum  and  Schulze,  by  a  method  similar  to  Fery's,  found 
apparent  variations  in  Bunsen  flame  temperatures  when  colored 
with  different  salts;  but  E.  Bauer,  using  the  same  method, 
showed  that  by  using  a  definite  part  of  the  flame  no  such  differ- 
ences exist  from  one  salt  to  another  nor  from  one  color  to  another. 
For  the  oxyhydrogen  flame  Bauer  finds  2240°  by  applying 
Planck's  law,  and  2200°  to  2300°  by  the  reversal  of  the  D  line, 
using  an  electric  arc  as  source  of  light  in  Fery's  method.  Bauer 
found  from  1660°  to  1850°  for  various  portions  of  the  Bunsen 
flame,  using  several  optical  methods. 

All  of  the  above  methods  assume  that  flames  are  nonlumi- 
nescent,  otherwise  the  results  obtained  are  too  high.  Absurd 
results  will  also  be  obtained  if  the  flames  are  colorless,  i.e.,  con- 
tain no  finely  divided  particles  heated  by  the  flame,  as  in  an 
open  Bunsen. 

Temperature  of  Glow-lamp  Filaments.  —  Since  the  observations 
of  Le  Chatelier  with  his  optical  pyrometer,  and  of  Lummer 
and  Pringsheim  making  use  of  the  Wien  relation  \mT  =  const., 
there  have  been  numerous  determinations  of  lamp  temperatures 
by  means  of  optical  pyrometers.  The  first  satisfactory  obser- 
vations for  a  series  of  lamps  were  made  by  Waidner  and  Burgess 
in  1906,  using  their  graphite-strip  method  of  comparison  (page 
330),  and  the  Holborn-Kurlbaum  instrument,  furnished  with 
red,  green,  and  blue  glasses  in  succession  before  the  eyepiece 
to  enable  estimations  of  true  temperature  to  be  made  from  the 
apparent  temperatures,  which  last,  of  course,  depend  upon  the 
selective  radiation  of  the  filament  surfaces.  They  found  that 
for  platinum  filaments  inclosed  in  an  evacuated  glass  bulb,  add- 
ing the  difference  in  temperature  between  the  blue  and  red  read- 
ings to  the  apparent  temperature  with  blue  light  when  sighted 
on  the  carbon  strip,  there  was  given  very  nearly  true  tempera- 
tures—for example,  1760°  C.  for  the  platinum  melting  point. 


OPTICAL   PYROMETER 


341 


Assuming  this  empirical  relation  to  hold  generally,  they  found  the 
following: 


NORMAL  BURNING  TEMPERATURES  OF  GLOW  LAMPS. 


Type  of  lamp. 

Watts  per 
candle 
power. 

Volts. 

Observed 
black-body 
temperatures 
(red). 

Maximum 
true  temper- 
ature. 

Minimum 
true  tem- 
perature. 

Carbon  

4-0 

50 

1710°  C. 

1800°  C. 

i'/55°C. 

Carbon-  

3-5 

118 

1760 

1850 

1805 

Carbon  

3-1 

118 

1860 

1950 

iQ°5 

Tantalum 

2    O 

no 

1865 

2OOO 

IQ5C 

Tungsten  

I  .O 

TOO 

2135 

2300 

22I5 

In  some  of  the  other  estimations  no  attempt  has  been  made 
to  correct  for  the  lack  of  blackness  of  the  filaments,  and  the 
results  appear  to  be  generally  too  low.  We  may  cite  the  follow- 
ing determinations: 


NORMAL  LAMP  TEMPERATURES  BY  VARIOUS  OBSERVERS. 


Observers. 

Grau 

Coblentz .  . 


Carbon. 

1660 

(-1785 

I  1570 


Tantalum.     Tungsten. 
1850 


Fery  

1780 

Pirani  

Joly                 ? 

1650 

to 

I 

JV^Ajr        .......       -\ 

1720 

1 

IQIO 
1670 


2000 


1740 


2O6O 

1810 


1875 


2080 


1810 


Method  and  remarks. 

Iridium   strip   and   Wanner  py- 
rometer. 

\mT=Cand  graphite  "black." 
(Xmr=C  and  platinum  "black." 
Temperature  obs.  of  Waidner 
(      and  Burgess  with  red  light. 

f  Combination  of  Wien  and  Ste- 
j      fan  laws;  assumes  W  behaves 
i      like  Pt.     Used  absorption  py- 
(^     rometer. 

Resistance  and  optical  measure- 
ments. 

(Total  photometric  (Nernst); 
other  methods  gave  lower  val- 
f     ues. 


These  figures  are  not  strictly  comparable,  as  the  ratings  are  not 

exactly  the  same;  roughly,  they  are  W  =  1.25  — ,  Ta  =  1.5 

c.p.  c.p. 

A  W 

and  carbon  =  3.5 

c.p. 


342 


HIGH  TEMPERATURES 


The  use  of  the  equation  Xm  T  =  C  (Coblentz)  is  questionable, 
as  the  form  of  the  energy  curves  of  lamp  filaments  is  not  that 
of  the  black  body. 

The  normal  burning  temperature  of  the  Nernst  filament  has 
been  measured  several  times,  ranging  from  the  absurdly  low 
result  of  Hartmann  of  1535°  obtained  with  a  thermocouple,  to 
the  value  2360°  of  Ingersoll  by  a  luminous-efficiency  method. 
Mendenhall  and  Ingersoll  found  that  rhodium  would  melt  on 
a  Nernst  filament  below  its  normal  burning,  and  that  iridium 
would  not,  which  places  this  temperature  at  about  2ioo°C.; 
an  application  of  Wien's  law  gave  them  2125°  C. 

Temperatures  within  Furnaces.  —  The  optical  pyrometer,  espe- 
cially in  its  forms  due  to  Wanner  and  to  Holborn  and  Kurlbaum, 
has  been  of  the  greatest  use  in  studying  very  high  tempera- 
ture phenomena,  including  the  formation,  modification,  and  dis- 
sociation of  many  chemical  products.  Besides  the  numerous 
melting-point  determinations  described  elsewhere,  we  may  men- 
tion as  illustrations  the  work  of  Nernst  and  his  associates  at 
Berlin  on  gaseous  dissociation  to  temperatures  above  2000°  C., 
carried  out  in  his  type  of  iridium  furnace;  of  Tucker  and  others 
at  Columbia  University  on  carborundum  and  other  furnace 
products;  of  Thompson  at  the  Mass.  Inst.  of  Technology  on  a 
series  of  chemical  reactions ;  of  Greenwood  and  of  Prim  at  Man- 
chester, using  a  carbon  vacuum  and  pressure  furnace,  on  boil- 
ing points  of  the  metals  and  on  the  temperature  of  formation  of 
many  chemical  substances.  In  all  of  the  above  investigations 
the  Wanner  pyrometer  was  used,  but  where  the  furnace  opening 
is  small,  as  is  usually  the  case,  there  is  advantage  in  using  an 
instrument  requiring  only  a  few  millimeters  area  to  sight  on,  as 
the  Holborn-Kurlbaum  type.  This  has  been  used  at  the  Reich- 
sanstalt  in  comparing  the  optical  and  gas  scales,  and  at  the 
Bureau  of  Standards  in  most  of  the  high-temperature  work 
there,  as  well  as  at  the  Geophysical  Laboratory.  Using  an 
Arsem  furnace  (Fig.  176),  Dr.  Kanolt  with  this  pyrometer  has 
been  able  to  measure  melting  and  freezing  points  of  salts,  alloys, 
and  minerals  to  temperatures  above  2100°  C.  by  taking  the 


OPTICAL  PYROMETER 


343 


heating  and  cooling  curves  and  making  use  of  the  latent  heat  of 
transformation.  A  few  tenths  of  a  gram  of  material  are  suffi- 
cient to  give  a  very  sharp  point  (see  Fig.  122). 


2100 


2000 


1900 


1800 


iroo 


1600 


1500 


1400- 


012 
Minutes 

Fig.  122.     Melting  Points  with  Optical  Pyrometer. 

Melting  points  of  microscopic  samples  may  also  be  obtained 
readily  with  the  Holborn-Kurlbaum  pyrometer  by  making  use 
of  the  known  departure  from  blackness,  or  the  emissivity,  of 
some  substance  such  as  platinum,  on  a  strip  of  which,  or  other 


344 


HIGH  TEMPERATURES 


suitable  material  such  as  indium,  carbon,  or  tungsten,  may  be 
placed  the  substance  whose  melting  point  is  sought. 

In  Fig.  123  is  shown  the  apparatus  of  Burgess  used  for  the 
determination  of  the  melting  of  points  of  the  iron  group  (Chap. 
XI)  in  hydrogen,  using  samples  of  the  order  of  o.ooi  mg.  melted 
on  a  platinum  strip  heated  by  a  delicately  adjustable  electric 
current.  The  container  is  of  brass  blackened  inside,  and  simul- 
taneous observations  are  taken  through  a  mica  window  of  the 


Fig.  123.     Apparatus  of  Burgess  for  Microscopic  Samples. 

melting  of  the  sample  with  a  microscope  and  of  the  temperature 
of  the  strip  with  the  pyrometer. 

Conditions  of  Use.  —  The  optical  pyrometer  using  mono- 
chromatic light,  by  reason  of  the  uncertainty  of  emissive  powers 
and  of  the  relatively  slight  sensibility  of  the  eye  for  comparisons 
of  luminous  intensities,  cannot  give  quite  as  accurate  results  as 
the  electric  methods,  although  the  accuracy  attainable,  since 
the  satisfactory  establishment  of  the  laws  of  radiation  through- 
out practically  the  attainable  temperature  range,  is  sufficient, 
as  we  have  seen,  when  proper  precautions  are  taken,  for  all 
industrial  and  most  scientific  needs.  The  range  of  this  py- 


OPTICAL  PYROMETER  345 

rometer  is  from  about  650°  C.  to  the  highest  attainable 
temperature. 

The  optical  or  radiation  pyrometer  is  peculiarly  well  adapted 
for  many  cases  in  which  other  methods  fail,  as  when  contact  with 
the  object  whose  temperature  is  sought  cannot  be  made  or  when 
for  any  reason  the  pyrometer  must  be  placed  at  a  distance ;  for 
example,  in  the  case  of  a  moving  body,  as  a  rail  passing  into  the 
rolling  mill;  in.  the  case  of  very  high  temperatures,  as  of  the 
crucible  of  the  blast  furnace  or  that  of  the  electric  furnace;  in 
the  case  of  isolated  bodies  radiating  freely  into  the  air,  as  flames 
or  wires  heated  by  an  electric  current  which  cannot  be  touched 
without  changing  their  temperature.  We  have  also  seen  that 
it  may  give  very  exact  results  in  such  cases  when  the  emissive 
properties  of  the  substances  sighted  upon  are  known,  as  is  often 
the  case. 

It  is  also  convenient  in  the  case  of  strongly  heated  furnaces, 
as  steel  and  porcelain  furnaces.  But  in  this  usage  care  must 
be  taken  to  guard  against  the  brightness  of  the  flames,  always 
hotter  than  the  furnace,  and  against  the  entry  of  cold  air.  The 
arrangement  with  the  closed  tube  described  in  connection  with 
the  heat-radiation  pyrometer  is  advisable  if  it  is  desired  to  ob- 
tain the  most  exact  results.  The  optical  pyrometer  has  the  incon- 
venience to  require  active  intervention  on  the  part  of  the  operator 
and  can  hardly  be  intrusted  to  a  workman  without  oversight, 
while  the  set-up  of  the  heat-radiation  pyrometer  may  be  made  so 
that  an  observation  reduces  to  a  reading  upon  a  scale.  The 
latter  pyrometer,  however,  is  the  more  subject  to  error  due  to 
lack  of  blackness,  flames,  and  furnace  gases. 

Some  Industrial  Uses.  —  The  several  forms  of  optical  pyrom- 
eter using  monochromatic  light  have  been  very  generally  intro- 
duced into  industrial  practice,  where  they  are  rendering  most 
useful  service,  and  for  many  operations  they  may  advantageously 
replace  the  eye  of  the  operator.  Practically  every  furnace  opera- 
tion can  be  controlled  by  this  type  of  pyrometer  with  great  pre- 
cision, with  a  resulting  saving  of  fuel  and  a  more  uniform  furnace 
product.  A  few  of  the  types  of  furnaces  for  which  such  pyrom- 


346  HIGH  TEMPERATURES 

eters  are  adapted  are  the  various  steel-melting  furnaces,  blast 
furnaces,  coke  ovens,  ceramic  kilns,  and  glass  weirs.  In  forging, 
annealing,  hardening,  and  similar  operations  on  steel,  and  in 
foundry  practice  in  general,  such  pyrometers  are  equally  useful. 
We  have  already  called  attention  (page  305)  to  some  industrial 
measurements  made  by  Le  Chatelier  with  his  optical  pyrometer. 
We  may  also  mention  some  determinations  with  the  Wanner 
pyrometer  on  a  battery  of  six  coke  ovens: 

Oven.  123456 

Over  the  retorts 1232     1264     1370    1464     1409     1436 

Just  over  generator . 1409     1397     1464     1397     1296     1264 

Fifth  flue 1126     1002     1112     1104     1096     1119 

Next  to  last  flue 992      982      918      932      970      932 

The  Morse  or  Holborn-Kurlbaum  type  may  be  sighted  on 
distant  objects  conveniently.  It  is  possible  to  set  up  such  an 
instrument  in  a  foundry  or  forging  shop  and  from  one  position 
measure  temperatures  of  several  furnaces,  of  pieces  under  the 
hammer,  and  of  metal  being  poured  into  and  from  ladles. 

Measurement  of  the  Relative  Intensity  of  Different  Radia- 
tions. —  It  is  on  this  principle  that  rests  the  eye  estimation  of 
temperatures  such  as  are  made  by  workmen  in  industrial  works. 
Numerous  attempts,  none  very  successful,  have  been  made  to 
modify  this  method  and  make  it  precise.  There  is  need  to  con- 
sider this  mainly  from  the  point  of  view  of  a  rough  control  over 
the  heating  of  industrial  furnaces.  Recently  a  modification  of 
this  method  has  been  devised  by  Nordmann,  which,  as  we  shall 
see,  is  of  interest  in  the  estimation  of  the  extremely  high  tem- 
peratures of  stars. 

Use  of  the  Eye.  —  Pouillet  made  a  comparison  of  the  colors  of 
incandescent  bodies  in  terms  of  the  air  thermometer.  The  table 
that  he  drew  up  is  reproduced  everywhere  to-day: 

POUILLET' S  COLOR   SCALE. 

First  visible  red 525°  Dull  orange 1100° 

Dull  red 700  Bright  orange 1200 

Turning  to  cherry 800  White 1300 

Cherry  proper 900  Brilliant  white 1400 

Bright  cherry 1000  Dazzling  white 1500 


OPTICAL  PYROMETER  347 

The  estimation  of  these  hues  is  very  arbitrary  and  varies  from 
one  person  to  another;  more  than  that,  it  varies  for  the  same 
person  with  the  exterior  lighting.  The  hues  are  different  by 
day  from  those  by  night;  it  is  thus  that  the  gas  flame,  yellow 
during  the  day,  appears  white  at  night.  It  is  only  in  the  reds 
that  any  accuracy  can  be  had  by  the  eye  method.  Workmen 
can  sometimes  guess  to  better  than  25°  C.  up  to  800°  C.  At 
1200°  errors  of  over  200°  will  be  made. 

Use  of  Cobalt  Glass.  —  One  may  exaggerate  the  changes  of 
hue  in  suppressing  from  the  spectrum  the  central  radiations,  the 
yellow  and  green  for  example,  so  as  only  to  keep  the  red  and  the 
blue.  The  relative  variations  of  two  hues  are  the  greater  the 
more  separated  they  are  in  the  spectrum;  now,  the  red  and  the 
blue  form  the  two  extremities  of  the  visible  spectrum. 

It  has  been  proposed  for  this  purpose  to  use  cobalt  glass,  which 
cuts  out  the  yellow  and  green,  but  lets  pass  the  red  and  blue.  It 
must  be  remembered  that  the  ratio  of  the  radiations  transmitted 
varies  with  the  thickness  of  the  glass  as  well  as  with  their  absolute 
intensities. 

Let  I  a  and  Ib  be  the  intensities  of  the  radiations  emitted,  ka 
and  kb  the  proportions  transmitted  by  the  glass  through  a  thick- 
ness i.  Through  a  thickness  e  the  proportion  transmitted  will 

be 


which  will  vary  with  e  in  all  cases  that  ka  is  different  from  kb. 

It  results  from  this  that  two  cobalt  glasses,  differing  in  thick- 
ness or  in  amount  of  cobalt,  will  not  give  the  same  results.  So 
that  if  the  cobalt  glass  habitually  used  is  broken,  all  the  training 
of  the  eye  goes  for  naught. 

Besides,  cobalt  has  the  inconvenience  of  having  an  insufficient 
absorbing  power  for  the  red,  which  predominates  at  the  more 
ordinary  temperatures  that  we  make  use  of.  It  would  be  possi- 
ble, without  doubt,  by  the  addition  of  copper  oxide,  to  augment 
the  absorbing  power  for  the  red. 

One   would   have   better   and   more  comparable  results   by 


348  HIGH  TEMPERATURES 

employing  solutions  of  metallic  salts  or  of  organic  compounds 
suitably  chosen.  But  few  trials  have  been  made  in  this  matter. 
Pyroscope  of  Mesure  and  Nouel.  —  It  is  known  that  by  placing 
between  two  nicols  a  plate  of  quartz  cut  perpendicularly  to  the 
axis,  a  certain  number  of  the  radiations  of  the  spectrum  are 
suppressed.  This  latter  is  then  composed  of  dark  bands  whose 
spacing  depends  on  the  thickness  of  the  quartz  and  the  position 
of  the  angle  of  the  nicols.  Mesure  and  Nouel  have  utilized  this 
principle  in  order  to  cut  out  the  central  portions  of  the  spectrum ; 
this  solution  is  excellent  and  preferable  to  the  use  of  absorbing 
media.  The  apparatus  (Fig.  124)  consists  essentially  of  a  polarizer 
P  and  an  analyzer  A,  whose  adjustment  to  extinction  gives  the 
zero  of  graduation  of  the  divided  circle  CC.  This  circle  is  gradu- 


Fig.  124.    Apparatus  of  Mesure  and  Nouel. 

ated  in  degrees  and  is  movable  before  a  fixed  index  7.  Between 
the  two  nicols  P  and  A  is  a  quartz  Q  of  suitable  thickness,  care- 
fully calibrated.  The  mounting  M  allows  of  its  quick  removal 
if  it  is  necessary  to  verify  the  adjustment  of  the  nicols  P  and  A. 
The  quartz  Q  is  cut  perpendicularly  to  the  axis.  A  lens  L  views 
the  opposite  opening  C  furnished  with  a  parallel-faced  plate 
glass  or,  where  desired,  with  a  diffusing  glass  very  slightly  ground. 

The  relative  proportions  of  various  rays  that  an  incandescent 
body  emits  varying  with  the  temperature,  it  follows  that  for  a 
given  position  of  the  analyzer  A  the  composite  tint  obtained  is 
different  for  different  temperatures. 

If  the  analyzer  is  turned  while  a  given  luminous  body  is  viewed, 
it  is  noticed  that  the  variations  of  coloration  are  much  more 
rapid  for  a  certain  position  of  the  analyzer.  A  very  slight  rota- 


OPTICAL  PYROMETER  349 

tion  changes  suddenly  the  color  from  red  to  green.  Now,  if  the 
analyzer  is  left  fixed,  a  slight  variation  in  the  temperature  of  the 
incandescent  body  produces  the  same  effect.  The  transmission 
hue  red-green  constitutes  what  is  called  the  sensitive  hue.  There 
are  then  two  absorptions,  one  in  the  yellow  and  the  other  in  the 
violet. 

This  apparatus  may  be  employed  in  two  different  ways.  First 
fix  permanently  the  analyzer  in  a  position  which  gives  the  sensi- 
tive hue  for  the  temperature  that  is  to  be  watched,  and  observe 
the  changes  of  hue  which  are  produced  when  the  temperature 
varies  in  one  direction  or  the  other  from  the  chosen  temperature. 
This  is  the  ordinary  method  of  use  of  this  instrument.  It  is 
desired  in  a  given  manufacturing  process  (steel,  glass)  to  make 
sure  that  the  temperature  of  the  furnace  rests  always  the  same; 
the  instrument  is  adjusted  once  for  all  for  this  temperature.  It 
suffices  to  have  but  a  short  experience  to  train  the  eye  to  appre- 
ciate the  direction  of  the  change  of  hue. 

The  inventors  have  sought  to  make  of  their  apparatus  a  meas- 
uring instrument;  this  idea  is  quite  open  to  debate.  In  theory 
this  is  easy;  it  suffices,  instead  of  having  the  analyzer  fixed, 
to  make  it  turn  just  to  the  securing  of  the  sensitive  hue  and  to 
note  the  angle  which  gives  the  position  of  the  analyzer.  But 
in  fact  the  sensitive  hue  is  not  rigorously  determinate  and  varies 
with  the  observer.  A  graduation  made  by  one  observer  will  not 
hold  for  another.  It  is  not  even  certain  that  the  same  observer 
will  choose  always  the  same  sensitive  hue.  At  each  temperature 
the  sensitive  hue  is  slightly  different,  and  it  is  impossible  to  re- 
member throughout  the  scale  of  temperatures  the  hues  that  were 
chosen  on  the  day  of  the  graduation.  There  is  even  considerable 
difficulty  to  recall  this  for  a  single  temperature. 

The  following  figures  will  give  an  idea  of  the  differences  which 
may  exist  between  two  observers  as  to  the  position  of  the  sensi- 
tive hue: 

Temper-          Angle  of  analyzer, 
ature.  (i)  (2) 

Sun 6000°  84  86 

Gas  flame 1680  65  70 

Red-hot  platinum 800  40  45 


350  HIGH  TEMPERATURES 

The  errors  in  the  estimation  of  temperatures  which  result 
from  the  uncertainty  of  the  sensitive  hue  will  thus  exceed  100°. 
With  observers  having  had  more  experience  the  difference  will 
be  somewhat  reduced,  but  it  will  remain  always  quite  large. 

Crova's  Pyrometer.  —  Crova  endeavored  to  give  to  the  method 
of  estimation  of  temperatures  based  on  the  unequal  variation 
of  different  radiations  of  the  spectrum  a  scientific  precision  by 
measuring  the  absolute  intensity  of  each  of  the  two  radiations 
utilized;  but  this  method,  from  the  practical  point  of  view,  does 
not  seem  to  have  given  more  exact  results  than  the  preceding 
ones. 

The  eye  is  much  less  sensitive  to  difference  of  intensity  than  to 
difference  of  hue,  so  that  there  is  no  advantage  in  making  use  of 
observations  of  intensity. 

Crova  compared  two  radiations, 

X  =  676  (red), 
X  =  523  (green), 

coming  from  the  object  studied  and  from  the  oil  lamp  used  as 
standard.  For  this  purpose,  by  means  of  a  variable  diaphragm, 
he  brings  to  equality  one  of  the  two  radiations  emanating  from 
each  of  the  sources,  and  measures  afterwards  the  ratio  of  the 
intensities  of  the  two  other  radiations. 

The  apparatus  is  a  spectrophotometer.  Placed  before  half 
the  height  of  the  flame  is  a  total  reflecting  prism,  which  reflects 
the  light  from  a  ground  glass,  lighted  by  the  radiations  from  an 
oil  lamp,  having  first  passed  through  two  nicols  and  a  diaphragm 
of  variable  aperture.  On  the  other  half  of  the  slit  is  projected 
by  means  of  a  lens  the  image  of  the  body  to  be  studied. 

Before  using  the  apparatus  it  is  necessary  to  adjust  the  ex- 
treme limits  of  the  displacement  of  the  spectrum  so  as  to  pro- 
ject successively  on  the  slit,  in  the  focus  of  the  eyepiece,  the  two 
radiations  selected  (X  =  676  and  X  =  523).  For  this  purpose 
there  is  interposed  between  the  two  crossed  nicols  a  4-mm. 
quartz  plate  which  reestablishes  the  illuminations;  for  extinction 
again,  the  analyzer  must  be  turned  115°  38'  for  X  =  523,  and 


OPTICAL  PYROMETER  351 

65°  52'  for  X  =  676.  The  instrument  is  then  so  adjusted  that 
the  dark  band  produced  by  the  quartz  is  situated  in  the  middle 
of  the  ocular  slit. 

The  apparatus  thus  adjusted,  in  order  to  make  a  measurement 
at  low  temperatures,  inferior  to  those  of  carbon  burning  in  the 
standard  lamp,  one  brings  to  equality  the  red  radiations  with 
the  diaphragm,  then,  without  touching  the  diaphragm  again, 
the  green  is  brought  to  equality  by  turning  the  nicol. 

The  optical  degree  is  given  by  the  formula 


V  =  1000  cos2  a 


denoting  by  a  the  angle  between  the  two  principal  sections  of 
the  nicols. 

For  higher  temperatures  the  operation  is  reversed;  one  brings 
first  the  green  to  equality  by  means  of  the  diaphragm,  then  the 
red  to  equality  by  a  rotation  of  the  analyzer.  The  optical  degree 

is  then  given  by  the  formula  N  =  I0°°  ,  and  the  rotation  vary- 

cos2  a 

ing  from  o°  to  90°,  the  optical  degrees  vary  from  1000°  to  infinity. 
This  method,  which  is  theoretically  excellent,  possesses  certain 
practical  disadvantages: 

1.  Lack  of  precision  of  the  measurements.    In  admitting  an 
error  of  10  per  cent  in  each  one  of  the  observations  relative  to 
the  red  and  green  radiations,  the  total  possible  error  is  20  per 
cent;  now,  between  700°  and  1500°  the  ratio  of  intensities  varies 
from  i  to  5:  this  leads  to  a  difference  of  ^  m  800°,  or  32°. 

2.  Complication  and  slowness  of  observations.     It  is  difficult 
to  focus  exactly  on  the  body  or  the  point  on  the  body  that  one 
wishes  to  study.    The  set-up  and  the  taking  of  observations 
sometimes  require  about  half  an  hour. 

3.  Absence  of  comparison  in  terms  of  the  gas  scale. 

The  a  priori  reason  that  had  led  to  the  study  of  this  method 
was  the  supposition  that,  in  general,  the  emissive  power  of  sub- 
stances was  the  same  for  all  radiations  and  that  consequently  its 
influence  would  disappear  by  taking  the  ratio  of  the  intensities 
of  the  two  radiations.  The  measurements  of  emissive  power 


352  HIGH  TEMPERATURES 

given  previously  prove  that  this  hypothesis  is  the  more  often 
inexact. 

Crova  also  suggested  that  the  upper  limit  of  the  spectrum  of 
an  incandescent  body  might  be  used  as  a  measure  of  its  tem- 
perature, and  Hempel  has  tried  this  method  with  a  special  form 
of  spectroscope,  using  a  luminescent  screen  for  observing  when 
the  upper  spectrum  limit  is  beyond  the  visible  radiations;  but, 
as  compared  with  the  photometric  and  radiation  pyrometers, 
only  crude  results  can  be  obtained. 

Use  of  the  Flicker  Photometer.  —  Lummer  and  Pringsheim 
have  shown  that  the  combination  of  a  spectral  apparatus  with 
a  flicker  photometer  permits  of  greatly  increasing  the  accuracy 
of  the  method  of  comparison  of  the  intensities  of  two  colors,  and 
also  permits  the  use  of  Wien's  law  (page  251)  in  the  calculation 
of  temperatures. 

Sighting  on  a  black  body  at  the  absolute  temperature  T,  and 
measuring  the  two  intensities  /i  and  72  corresponding  to  the 
wave  lengths  Xi  and  \2,  we  have  from  Wien's  law: 


i/i         i     X2   , 
log  -1  =  5  log-2  + 

/2  AI 


in  which  T  is  the  only  unknown.     Thiirmel  has  shown  that  the 
Purkinje  effect  does  not  vitiate  the  observations,  and  that  results 
good  to  better  than  2  per  cent  can  be  obtained,  and  that  an 
observer  will  repeat  his  readings  within  this  limit  of  error. 
Here  are  some  of  Thiirmers  observations  on  a  black  body: 

TEMPERATURE  WITH  SPECTRAL  FLICKER  PHOTOMETER. 

i  --  Temperature  with  --  •» 

Ratio  of  wave  lengths.         apparatus.         Thermocouple. 
660-480  1502°  1477° 

660-500  1489  ____ 

660-480  1742  1698 

660-500  i  703  .       .... 

Sighting  on  other  objects  than  a  black  body  will  give  incorrect 
temperatures,  usually  low,  due  to  the  difference  in  shape  of  the 
intensity  curve  from  that  of  a  black  body,  and  on  account  of 


OPTICAL  PYROMETER  353 

the  varying  value  of  the  absorption  coefficient  with  wave  length 
from  one  substance  to  another  (see  page  256). 

Stellar  Pyrometers.  —  Of  recent  years  there  has  been  an  in- 
creasing interest  among  astronomers  in  the  determination  of  the 
physical  characteristics  of  the  stellar  bodies,  resulting  in  the 
development  and  modification  of  physical  instruments  suited  to 
their  needs.  Assuming  that  the  ratio  of  intensities  of  two  spectral 
colors,  red  and  blue  for  example,  varies  according  to  Planck's  law 
(page  251)  for  the  terrestrial  and  celestial  bodies  sighted  upon, 
M.  Nordmann  has  recently  constructed  a  heterochrome  photom- 
eter and  used  it  for  the  estimation  of  effective  stellar  tempera- 
tures. 

With  this  apparatus,  which  is  still  in  a  somewhat  crude  state 
of  development,  measurements  are  made,  in  the  various  parts  of 
the  spectrum,  of  the  brightness  of  the  star  under  observation 
referred  to  that  of  an  artificial  star  realized  by  means  of  a  second- 
ary electric  standard,  interchanging,  in  the  path  of  rays  common 
to  the  two  stars,  a  series  of  monochromatic  liquid  screens. 

Consider  measurements  with  red  and  blue  light. 

If  r,  7",  T" ,  .  .  .  are  known  temperatures  of  definite  light 
sources,  given,  for  example,  by  electric  furnaces  and  the  carbon 
arc,  and  if  R,  R',  R",  ...  and  B,  B' ',  B" ,  ...  are  the  corre- 
sponding intensities  of  the  images  as  measured  through  red-  and 
blue-light  filters  respectively  by  means  of  the  stellar  photometer, 

7?  i 

then,  according  to  Planck's  law,  the  relation  log  —  vs.  —  is  a 

B        T 

straight  line.  With  the  apparatus  once  standardized  at  known 
temperatures  therefore,  it  is  only  necessary  to  measure  the  red 
and  blue  intensities  for  any  light  source  as  a  star,  in  order  to 
find  its  apparent  or  black-body  temperature.  The  temperature 
found  will  approach  the  true  temperature  the  more  nearly  the 
spectral-energy  curve  of  the  star  approaches  that  of  the  black 
body.  In  the  form  of  apparatus  used  by  Nordmann,  it  is  also 
necessary  to  correct  for  the  shift  of  equivalent  wave  length  with 
temperature  of  the  monochromatic  screens  used.  This  could  be 
avoided  by  transforming  the  apparatus  into  a  spectrophotom- 


354 


HIGH  TEMPERATURES 


eter  in  much  the  same  manner  as  Henning's  spectral  pyrometer 
eliminates  the  colored  glasses  of  the  Holborn-Kurlbaum  instru- 
ment. 

Some  of  the  results  found  by  M.  Nordmann  for  effective  stellar 
temperatures  absolute  are  as  follows: 


p  Persei 2870° 

f  Cephii 4260 

Sun 5320 

7  Cygni 5620 


Polaris. 8,200° 

a  Lyrae 12,200 

d  Persei 18,500 

X  Tauri  40,000 


M.  Fery  has  realized  a  form  of  stellar  pyrometer  which  elimi- 
nates the  use  of  colored  screens.  The  principle  of  this  apparatus, 
which  is  based  on  Wien's  displacement  law  (page  249),  consists 
in  modifying  the  color  of  a  comparison  lamp  by  changing  the 


Fig.  125.     F6ry  Spectral  Pyrometer. 

ratio  of  the  monochromatic  intensities  which  it  emits,  so  as  to 
match  this  color  with  that  of  the  star  whose  temperature  is  to 
be  measured. 

In  order  to  realize  this  principle,  the  light  from  the  lamp  L 
(Fig.  125),  after  passing  through  the  slit  F,  is  dispersed  by  the 
direct- vision  prism  P,  and,  by  means  of  the  lenses  L  and  LI, 
forms  a  spectrum  in  the  plane  of  a  diaphragm  D,  to  which  we 
shall  return.  A  third  lens  LZ  forms,  on  the  half -silvered  mirror 
G,  a  white  or  undispersed  image  of  the  face  of  the  prism  P. 

The  light  from  the  star  is  concentrated  by  a  telescope  objective, 
whose  tube  is  shown  at  T,  and  an  image  of  the  star  is  formed  on 
the  nonsilvered  part  of  the  mirror  G,  and  may  be  examined  by 


OPTICAL   PYROMETER  355 

an  ocular  simultaneously  with  the  adjacent  luminous  area  due 
to  light  from  the  comparison  lamp. 

Fig.  126  gives  the  details  of  the  diaphragm  D  of  Fig.  125. 
The  spectrum  is  formed  between  the  two  screens  V  and  V'\  this 
last  is  semicircular  and  may  be  turned 
about  A  as  an  axis.  This  rotation  of 
V  causes  the  ratio  of  the  intensities  of  J 


the  extreme  red  and  blue  rays  to  vary      u     ^l-'A 
and  gives  to  the  field  as  projected  by  the       \  V' 

lens  L%  (Fig.  125)  the  desired  color. 

Both  of  the  above  types  of  apparatus 
may  be  calibrated  for  the  lower  tern.  Fig.  126.    Detail  of 

i  p  *     *   •      r  Diaphragm, 

peratures  by  means  of  an  electric  fur- 
nace, and  for  the  higher  stellar  temperatures  by  taking  the  arc 
and  sun  temperatures  as  fixed  points. 

Spectrophotometric  measurements  of  the  apparent  tempera- 
tures of  the  sun  and  109  stars  have  been  made  by  Wilsing  and 
Scheiner,  using  as  comparison  source  an  incandescent  lamp  cali- 
brated against  a  black  body.  Light  of  five  wave  lengths  was 
used,  and  the  observations  were  reduced  in  terms  of  Planck's 
law,  using  an  equation  similar  to  that  of  p.  255.  Here  are  some 
of  their  results,  assuming  c%  —  14,600. 

WILSING  AND  SCHEINER'S  STELLAR  TEMPERATURES  (ABS.). 

Sun 5130  to  5600°       a  Leoni  8700° 

f  Pegasi 7900        a  Lyrae 8100 

a  Pegasi 8700        7  Geminorum 6900 

t( 

We  shall  return  to  the  question  of  the  sun's  apparent  tem- 
perature in  Chapter  XI. 

Action  of  Light  on  Selenium.  —  It  has  been  known  for  a  long 
time  that  light  incident  upon  selenium  changes  the  electric  re- 
sistance of  the  latter,  and  pyrometers  based  on  this  principle 
have  been  devised.  Light  from  an  incandescent  source  whose 
temperature  is  sought  falls  upon  a  selenium  cell  forming  part  of 
an  electric  circuit  in  which  are  a  battery  and  ammeter.  As  the 
light  varies  in  intensity  due  to  changes  in  temperature,  the  re- 


356  HIGH  TEMPERATURES 

sistance  of  the  selenium  varies,  and  the  indications  of  the  ammeter 
may  be  empirically  calibrated  in  terms  of  temperature.  As  sele- 
nium is  quite  insensible  to  the  invisible  heat  waves,  the  lower 
limit  of  this  method  is  above  incandescence.  Selenium  also 
requires  some  time  to  recover  its  original  resistance  after  being 
acted  upon  by  light,  and  this  lag  might  prove  troublesome.  As 
a  dial  instrument  is  used,  the  method  could  readily  be  made 
recording. 


CHAPTER  IX. 
VARIOUS  PYROMETRIC  METHODS. 

WHILE  some  of  the  several  types  of  pyrometer  which  we  have 
described  in  the  preceding  chapters  have,  by  a  process  of  elimina- 
tion, become  generally  recognized  as  meeting  most  requirements 
for  high-temperature  measurements,  scientific  and  industrial, 
there  nevertheless  remain  several  methods,  some  of  which  are 
useful  in  special  fields  of  investigation  or  practice,  and  others 
mark  some  important  development  in  the  history  of  pyrometry. 
We  shall  mention  briefly  a  few  of  these  methods. 

Wedgwood's  contraction  pyroscope,  the  oldest  among  such 
instruments,  presents  to-day  hardly  more  than  an  historic  inter- 
est, for  its  use  has  been  almost  entirely  abandoned.  It  utilizes 
the  permanent  contraction  assumed  by  clayey  matters  under  the 
influence  of  high  temperature.  This  contraction  is  variable  with 
the  chemical  nature  of  the  paste,  the  size  of  the  grains,  the  com- 
pactness of  the  wet  paste,  the  time  of  heating,  etc.  In  order  to 
have  comparable  results,  it  would  be  necessary  to  prepare  simul- 
taneously, under  the  same  conditions,  a  great  quantity  of  cylinders, 
whose  calibration  would  be  made  in  terms  of  the  gas  thermometer. 
Wedgwood  employed  cylinders  of  fire  clay,  baked  until  dehy- 
drated, or  to  600°;  this  preliminary  baking  is  indispensable,  if 
one  wishes  to  avoid  their  flying  to  pieces  when  suddenly  sub- 
mitted to  the  action  of  fire.  These  cylinders  have  a  plane  face 
on  which  they  rest  in  the  measuring  apparatus,  so  as  always  to 
face  the  same  way  (see  the  frontispiece).  The  contraction  is 
measured  by  means  of  a  gauge  formed  by  two  inclined  edges; 
two  similar  gauges  of  6  inches  in  length,  one  an  extension  of  the 
other,  are  placed  side  by  side;  at  one  end  they  have  a  maximum 
separation  of  0.5  inch,  and  at  the  other  a  minimum  separation 
of  0.3  inch.  Longitudinally  the  divisions  are  of  0.05  inch;  each 

357 


358  HIGH  TEMPERATURES 


division  equals  %\-$  of  T%  of  an  inch,  or  yaV^  inch,  which  corre- 
sponds to  a  relative  contraction  of  y^Vo  "*"  iV  =  eTo"  m  terms  of 
the  initial  dimensions. 

We  then  have  the  following  relation  between  the  Wedgwood 
degrees  and  the  linear  contraction  per  unit  of  length: 

Wedgwood  ...........   o      30        60        90        120       150       180       210       240 

Contraction  ..........  o    0.05     o.io    0.15     0.20    0.25     0.30    0.35     0.40 

Le  Chatelier  has  made  experiments  to  determine  the  degrees 
of  the  Wedgwood  pyrometer  in  terms  of  the  scale  of  the  air 
thermometer  by  making  use  of  clayey  substances  of  different 
kinds,  and  in  the  first  place  of  the  cylinders  from  an  old  Wedg- 
wood pyrometer  of  the  Ecole  des  Mines.  The  contraction  which 
accompanies  the  dehydration  is  quite  variable  with  the  nature 
of  the  pastes.  In  these  experiments  the  time  of  heating  was  half 
an  hour. 

Centigrade  temperature  .............  600°  800°  1000°  1200°  1400°  1550° 

Wedgwood  ..........................  o  4  15  36  90  132 

Argile  de  Mussidan  .................  o  2  14  36  78  120 

Limoges  porcelain  ...................  o  o  2  21  88  91 

Faience  de  Choisy-le-Roi  ............  o  2  5  12  48  75 

Faience  de  Nevers  ..................  o  o  o  32  Melted  Melted 

Kaolin  .............................  04  12  15  55  118 

Clay  ..................  25  ) 

Titanic  acid  ..........  75}  ..........  o  4  9  iQ  123  160 

This  table  shows  how  variable  are  the  observations;  it  is 
impossible,  consequently,  to  compare  the  old  measurements  of 
Wedgwood  and  of  his  successors,  because  the  manufacture  of  the 
cylinders  has  varied  with  the  course  of  time. 

Wedgwood  had  given  a  graduation  made  by  a  process  of  extrap- 
olation which  he  has  not  explained,  —  a  graduation  according  to 
which  he  attributed  10,000°  C.  to  130°  of  his  pyrometer,  which 
corresponds  to  about  1550°.  One  might  still  seek  to  reestablish 
the  graduation  by  utilizing  the  determinations  of  the  fusing 
points  of  the  metals  made  by  Wedgwood,  but  the  results  are  too 
discordant  to  warrant  any  definite  conclusion.  According  to 
Wedgwood,  copper  would  be  more  fusible  than  silver,  iron  would 
not  be  far  removed  from  silver;  it  is  probable  that  these  obser- 
vations were  made  with  very  impure  metals,  or  at  any  rate  were 


VARIOUS  PYROMETRIC   METHODS  *  359 

made  with  metals  much  oxidized  before  their  fusion.  In  any 
case  the  cylinders  which  he  made  use  of  in  his  first  experiments 
assume  a  much  greater  contraction  than  those  of  the  pyrometer 
of  the  School  of  Mines  whose  graduation  was  given  above.  One 
might  with  considerable  reserve  indicate  the  following  graduation 
for  measurements  made  with  the  first  cylinders  employed  about 
the  year  1780: 

Wedgwood  degrees o          15  30        100         140 

Centigrade  degrees 600        800        1000       1200       1400 

The  preparation  of  the  cylinders  was  a  most  care-taking  oper- 
ation. Molded  in  soft  paste,  they  were  necessarily  somewhat 
irregular.  After  the  first  baking  they  had  to  be  trimmed  to 
bring  them  to  a  uniform  size.  To-day,  in  several  pottery  works 
where  the  method  is  still  employed,  a  much  greater  regularity 
is  obtained  by  using  a  very  dry  paste,  5  per  cent  of  water  for 
example,  molding  it  under  great  pressure,  about  100  kg.  per 
square  centimeter,  in  molds  of  turned  steel.  The  precision  of 
the  measurements  is  increased  by  augmenting  the  diameter,  to 
50  mm.  for  example.  It  is  necessary  at  the  same  time  to  reduce 
the  thickness  to  about  5  mm.,  in  order  that  the  compression  be 
uniform  throughout  the  mass. 

This  apparatus  cannot  be  recommended  in  any  instance  as  a 
true  pyrometer,  serving  indirectly  to  evaluate  temperatures  in 
terms  of  the  air-thermometer  scale.  The  graduation  is  laborious 
and  can  only  be  made  by  means  of  the  intermediary  of  another 
pyrometer;  the  use  of  fixed  points  is  riot  adapted  for  this  gradua- 
tion because  the  curve  of  contraction  of  clay  in  function  of  the 
temperature  is  too  irregular  for  two  or  three  points  to  determine 
it;  in  no  case  do  the  indications  of  this  instrument  possess  any 
considerable  precision. 

But  as  simple  pyroscope,  that  is  to  say,  as  an  apparatus  to 
indicate  merely  the  equality  or  inequality  of  two  temperatures, 
the  Wedgwood  pyrometer  is  very  convenient.  It  has  the  advan- 
tage of  costing  almost  nothing  and  it  is  easy  to  use  and  within 
the  comprehension  of  any  workman.  It  seems  to  be  particularly 
recommendable  for  certain  ceramic  industries,  in  which  the  ordi- 


HIGH  TEMPERATURES 


nary  pastes  found  there  may  be  used  to  make  the  contraction 
cylinders.  It  is  necessary  for  this  that  the  normal  baking  of 
these  pastes  be  stopped  at  a  temperature  comprised  within  the 
period  of  rapid  contraction.  This  is  the  case  with  fine  faience 
and  with  ordinary  earthenware.  That  would  not  be  the  case, 
however,  for  stanniferous  faience  nor  for  porcelain,  because  the 
baking  of  the  first  is  stopped  before  the  beginning  of  the  con- 
traction, and  that  of  the  second  after  its  finish. 

Expansion  of  Solids. — Some  of  the  earliest  forms  of  indicating 
pyrometers  were  based  on  the  relative  expansion  of  two  metals, 
or  of  a  metal  and  graphite  or  fire  clay. 
Some  of  these  instruments  have  had 
and  still  enjoy  a  very  wide  use  both  in 
Europe  and  America,  often  under  the 
name  mechanical  thermometers  for  the 
lower-range  instruments,  and  some  of 
them  are  suitable  for  certain  industrial 
processes  not  requiring  exact  temperature 
determination  or  control.  A  common 
form  of  dial  instrument  is  shown  in 
Fig.  127.  A  tube  of  iron  incloses  a  rod 
of  graphite,  and  their  differential  expan- 
sion with  change  in  temperature  is  com- 
Imunicated  by  levers  to  a  pointer  turning 
over  a  dial  graduated  in  degrees.  The 
upper  limit  of  these  instruments  is  about 
800°  C.  (1500°  F.),  but  they  deteriorate 
rapidly  when  used  at  the  higher  tem- 
peratures. Their  indications  change  with 
time,  due  to  changes  produced  in  the 
materials  by  continued  heatings.  Cor- 
recting the  zero  of  such  an  instrument, 
which  should  be  done  frequently,  does  not  completely  correct 
the  rest  of  the  scale,  as  the  expansion  properties  of  the  two 
materials  change  differently  with  heating.  Varying  depths  of 
immersion  will  also  change  the  readings. 


Fig.  127.    Expansion 
Pyrometer. 


VARIOUS   PYROMETRIC   METHODS 


The  Joly  Meldometer.  —  A  modified  form  of  this  instrument 
was  previously  mentioned  (page  271).  As  in  its  usual  form  it 
may  be  of  great  service  to  chemists,  mineralogists,  and  others  in 
determining  the  melting  points  and  identification  of  minute  speci- 
mens of  minerals,  salts,  metals,  and  alloys,  a  further  description 
may  be  of  interest. 


H 


Platinum-Strip 


Air  Current  Shield 


Fig.  128.    Joly's  Meldometer. 

A  platinum  strip  (Fig.  128)  10  cm.  long,  4  mm.  wide,  and  0.02 
mm.  thick  is  held  between  two  clamps  C,  C,  and  kept  under  a 
slight  tension  by  the  spring  s.  A  storage-battery  current  con- 
trolled by  a  small  step  rheostat  R  is  sent  through  the  platinum 
strip  whose  length  at  any  instant  is  given  by  the  micrometer 
screw  M,  whose  contact  is  made  appreciable  by  the  closing  of 
the  circuit  of  an  electric  bell.  The  platinum  strip  is  calibrated 
preferably  by  means  of  salts  of  known  melting  points,  as  KNO3 
(399°  C.),  KBr  (730°),  NaCl  (800°),  and  K2SO4  (1060°).  Metals. 


362  HIGH  TEMPERATURES 

may  also  be  used,  but  they  tend  to  deteriorate  the  platinum. 
The  upper  limit  of  the  instrument  is  about  1500°  C.,  the  Pd  point 
being  obtainable  with  difficulty.  Permanent  elongation  sets  in 
somewhat  before  this  point  is  reached.  The  gold  point  (1063°  C.) 
can  be  determined  to  2°  C.,  and  only  a  few  moments  are  required 
for  an  observation. 

To  take  an  observation,  a  speck  of  the  specimen  whose  melt- 
ing point  is  sought  is  placed  on  the  middle  of  the  strip  under  a 
low-power  microscope  magnifying  about  twenty-five  times.  The 
current  is  increased  and  at  the  instant  of  melting,  as  observed 
with  the  microscope,  the  micrometer  is  set  to  make  contact  and 
read;  when  by  interpolation,  most  conveniently  made  graphically, 
the  temperature  is  found  corresponding  to  the  length  of  strip 
observed.  This  instrument  gives  a  nearly  but  not  quite  linear 
relation  between  length  of  strip  and  temperature. 

High-range  Mercury  Thermometers.  —  Although  mercury 
boils  normally  at  about  356°  C.,  yet  this  liquid  subjected  to  high 
pressure  may  be  kept  from  boiling  and,  suitably  inclosed,  may  be 
used  as  thermometric  substance  to  much  higher  temperatures. 
Compressed  under  an  atmosphere  of  some  inert  gas,  as  nitrogen 
or  carbonic  acid,  'and  inclosed  in  a  very  hard  glass,  the  mercury 
thermometer  can  be  used  up  to  550°  C.  (1000°  F.).  When  a  ther- 
mometer designed  for  only  moderate  temperatures,  200°  C.  or  less, 
is  sealed  off  gas  free,  there  will  be  distillation  of  the  mercury  into 
the  colder  parts  of  the  bore  unless  the  column  projects  sufficiently 
above  the  heated  region  or  the  whole  thermometer  is  immersed. 

There  are  two  methods  of  producing  the  necessary  pressure 
within  the  bore  to  prevent  distillation  and  boiling  of  the  mercury. 
In  the  one,  there  is  a  small  bulb  at  the  top  of  the  bore,  and  the 
thermometer  is  sealed  off  at  atmospheric  pressure  with  the  mer- 
cury at  ordinary  temperature;  in  the  other,  there  is  a  large  upper 
bulb,  and  the  sealing  off  is  done  at  increased  pressure,  making 
use  of  an  auxiliary  bulb.  The  second  construction  is  preferable, 
as  the  internal-pressure  change  with  rise  of  temperature,  and  con- 
sequent deformation  of  the  main  bulb  containing  mercury,  is 
much  less  than  with  the  first. 


VARIOUS  PYROMETRIC   METHODS 

Due  to  deformations  in  the  glass,  and  consequent  changes  in 
readings,  all  high-range  mercury  thermometers  should  be  fur- 
nished with  some  fixed  point,  preferably  the  ice  point.  This 
permits  controlling  conveniently  the  behavior  of  the  thermom- 
eter due  to  changes  in  the  volume  of  the  bulb  after  the  instru- 
ment has  been  calibrated.  The  bulbs  of  such  thermometers 
should  be  carefully  annealed,  before  filling,  at  a  temperature 
higher  than  the  instrument  is  to  be  used,  and  the  thermometer 
should  also  be  annealed  after  it  is  made  and  allowed  to  cool 
slowly,  otherwise  considerable  and  irregular  changes  in  its  in- 
dications will  occur,  amounting  to  several  degrees.  It  is  also 
advantageous  to  heat  and  cool  slowly  the  thermometer  a  great 
many  times  before  testing  and  using  it.  The  zero  reading  of 
such  a  thermometer  should  be  taken  after  every  observation  in 
work  of  precision.  If  a  considerable  length  of  stem  emerges  into 
the  air  when  taking  a  reading,  a  very  considerable  error,  25°  C. 
or  so,  may  be  introduced  at  high  temperatures  due  to  the  differ- 
ence in  temperature  of  the  bulb  and  stem.  This  "stem  correc- 
tion "  varies  slightly  from  one  kind  of  glass  to  another  and  is 
very  nearly: 

Stem  correction  =    0.00016  -  n  •  (T  —  f)°  C., 
=  0.000088-  n-(T  — /)°F., 

where  n  =  number  of  degrees  emergent  from  bath; 
T  =  temperature  of  bath; 
/  =  mean  temperature  of  the  emergent  mercury  column 

determined  by  some  auxiliary  means,  as  the  faden 

thermometer  of  Mahlke. 

Among  the  thermometric  glasses  for  the  construction  of  high- 
range  instruments,  and  the  upper  limits  to  which  they  may  be 
used  safely,  are:  Jena  i6m  normal,  Corning  normal,  and  the 
French  verre  dur,  which  reach  450°  C.  or  somewhat  higher; 
Jena  59™,  a  borosilicate  glass,  although  sometimes  graduated  to 
550°  C.,  should  not  be  used  over  520°  C.;  with  special  grades 
of  combustion  tubing  570°  C.  may  be  reached.  If  after  proper 
annealing  and  preliminary  heat  treatment  the  zero  of  a  ther- 
mometer falls,  it  is  being  used  at  too  high  temperatures. 


364  HIGH  TEMPERATURES 

Thermometers  which  are  to  be  used  as  high-temperature  pri- 
mary standards,  or  instruments  which  reproduce  in  themselves  the 
temperature  scale,  should  have  both  the  ice  and  steam  points, 
which  permits  calibrating  the  instrument  in  terms  of  the  funda- 
mental interval  o°  to  100°  C.  Due  to  the  fact  that  the  mercury- 
in-glass  expansion  varies  from  glass  to  glass,  and  is  also  different 
for  all  of  them  from  the  gas  expansion  on  which  the  temperature 
scale  is  based,  it  is  necessary  to  apply  a  correction  to  reduce  the 
readings  of  a  mercury-in-glass  thermometer  to  the  gas  scale, 
unless  the  thermometer  was  originally  "  pointed  "  in  terms  of 
this  scale.  The  relation  between  the  scales  given  by  Jena  glasses 
and  the  gas  scale  is  shown  in  the  following  table: 

VARIATION  FROM  GAS  SCALE  OF  JENA-GLASS  THERMOMETERS. 


Gas  scale. 

Jena  i6m              Gas  scale. 

Jena  59IU 

o 

o 

O 

o 

IOO 

IOO.OO 

IOO 

IOO.O 

ISO 

149  .  90 

2OO 

200.7 

200 

200.04 

300 

304.1 

220 

22O.  21 

325 

330-9 

240 

24O  .  46 

350 

358.1 

260 

260.83 

375 

385.4 

280 

281.33 

400 

412.3 

300 

301.96 

425 

440.7 

450 

469.1 

475 

498.0 

500 

527.8 

If  the  bore  of  the  thermometer  is  irregular,  it  should  be  calibrated 
by  the  use  of  a  5o-degree  or  loo-degree  thread. 

Ordinary  high-temperature  thermometers  are  tested  most  con- 
veniently by  comparison  with  a  standard,  or  by  taking  readings 
at  a  series  of  known  temperatures.  High- temperature  thermom- 
eters for  a  given  limited  range  are  kept  of  a  reasonable  length  of 
stem  and  at  the  same  time  with  an  open  scale  by  the  insertion 
of  intermediary  bulbs  which  eliminate  the  undesired  portions  of 
the  scale.* 

Therrnometric  glasses  and  high-temperature  thermometers  are  dis- 
cussed in  Hovestadt's  "  Ja'ener  Glas  "  (in  German  and  English),  Mathias' 
"  Les  Modifications  Permanentes  du  Verre,"  and  in  the  publications  of  the 
Bureau  of  Standards.  Guillaume's  ' '  Thermome'trie  de  Precision ' '  describes 
details  of  calibration  and  manipulation. 


VARIOUS   PYROMETRIC   METHODS  365 

The  glass  of  mercury  thermometers  has  been  successfully  re- 
placed by  quartz,  which  is  almost  an  ideal  thermometric  envelope, 
possessing  an  insignificant  expansion  and  no  appreciable  zero 
lag,  and  capable  of  being  used  at  very  high  temperatures.  Such 
mercury-in-quartz  thermometers  are  now  constructed  by  Siebert 
and  Kuhn,  and  are  graduated  to  about  700°  C. 

Dufour  has  tried  to  substitute  tin  for  mercury-in-quartz  ther- 
mometers, thereby  attaining  a  temperature  of  over  1000°  C. 
Such  thermometers  have  not  yet,  however,  come  into  use.  It 
is  a  difficult  matter,  not  yet  satisfactorily  solved,  to  find  a  sub- 
stance suitable  to  use  as  thermometric  fluid  in  quartz  at  high 
temperatures. 

Fusing-point  Pyrometry.  — As  long  ago  as  1827,  Prinsep  pro- 
posed to  compare  temperatures  by  means  of  the  fusing  points  of 
certain  metals  and  alloys.  But  the  nonoxidizable  metals  are 
not  numerous  and  all  are  relatively  very  costly:  silver,  gold, 
palladium,  platinum.  Use  has,  however,  been  made  sometimes 
of  these  metals  and  their  alloys,  in  admitting  that  the  fusing 
point  of  a  mixture  of  two  substances  is  the  arithmetical  mean 
of  the  points  of  fusion  of  the  components,  which  is  not  quite 
exact.  The  use  of  these  alloys  is  entirely  abandoned  to-day, 
and  with  reason. 

In  a  sense,  this  method  of  pyrometry  may  be  said  to  be  still  in 
use,  since  the  temperature  scales  of  the  several  standardizing 
laboratories  are  practically  defined  by  the  freezing  temperatures 
of  pure  metals. 

By  making  use  of  metallic  salts,  among  which  a  great  number 
may  be  heated  without  alteration,  one  might  construct  a  scale 
of  fusing  points  whose  use  would  be  often  very  convenient;  but 
this  work  is  not  yet  done,  at  least  not  in  a  sufficiently  precise 
manner.  To  the  separate  salts  may  be  added  their  definite 
combinations  and  their  eutectic  mixtures  which  have  perfectly 
definite  fusing  points.  But  any  mixture  whatever  of  two  salts 
cannot  be  taken,  because  in  general  the  solidification  takes  place 
throughout  a  large  interval  of  temperature  and  in  a  progressive 
manner. 


366  HIGH  TEMPERATURES 

In  some  metallurgical  operations,  it  is  often  necessary  to  be 
certain  that  objects  are  heated  above  some  definite  temperature. 
Salt  baths  of  known  freezing  points,  and  of  materials  not  attack- 
ing the  metals  used,  serve  excellently  both  for  heating  such 
objects  and  automatically  giving  the  minimum  temperature  allow- 
able. 

We  may  cite  in  this  connection  the  investigations  of  Brearley 
and  Morewood  and  of  Grenet  on  pure  salts  and  eutectic  and  iso- 
morphous  mixtures  suitable  for  this  purpose.  For  the  heat  treat- 
ment of  steels,  Grenet  recommends  the  following  series  of  salts : 

GRENET'S  SERIES  OF  SALTS  FOR  HEAT  TREATMENT 
OF  STEELS. 

Melting  point.  Melting  point. 

K2S04 1070°  C.     KC1 775°  C. 

BaCl2 955  KBr 730 

Na2SO4 865  KI 682 

5K2S04+5Na2S04...  850  5.8 KC1+4.2  NaCl... .  655 

3K2SO4+7Na2SO4...  830  3NaCl+7KBr 625 

2K2SO4+8Na2S04:..  825  Ba(NO3)2 600 

Na2CO3 810  Ca(NO3)2 550 

NaCl 800 

The  uncertainty  of  our  knowledge  of  the  numerical  values  of 
the  melting  points  of  some  of  the  salts  is  illustrated  in  the  table 
on  next  page  and  in  Chapter  XI. 

It  would  be  worth  while  to  carry  out  a  careful  series  of  deter- 
minations of  the  melting  points  of  these  and  other  salts,  using 
the  care  and  refinements  of  method  that  have  been  employed  in 
recent  work  on  metals,  and  employing  large  quantities  of  salt, 
300  to  1000  grms. 

"  Sentinel  pyrometers  "  and  pastes,  such  as  those  of  Brearley 
(The  Amalgams  Company,  Sheffield),  are  also  useful  in  certain 
operations.  The  former  are  cast  in  the  form  of  small  cylinders 
from  molecular  mixtures  of  salts.  For  temperatures  below  500°  C. 
they  are  inclosed  in  glass  tubes  and  therefore  last  indefinitely, 
and  for  higher  temperatures  are  placed  in  saucers  (Fig.  129). 
Two  such  "  sentinels  "  may  be  used,  for  example,  to  control  a 
furnace  within  any  given  temperature  range,  the  one  being  liquid 
and  the  other  solid.  The  paste,  made  from  salts  mixed  with 


VARIOUS   PYROMETRIC   METHODS 
MELTING  POINTS  OF   SALTS. 


367 


Date. 

1896. 

1894. 

1903. 

1905. 

Authors. 

Ramsay  and 
Eumorfopoulos. 

McCrae. 

Ruff  and  Plato. 

Huttner  and 
Tammann. 

Method. 

Meldometer. 

Thermoelectric. 

Thermoelectric. 

Thermoelectric. 

Calibration 
data. 

KN03=339 

K2S04=Au+7° 
=  1052. 

Diphenylamine 
=304 
SBP=445 

Au=IO?2. 

Reichsanstalt 
scale. 

Sb=  630.6 
Au=io64. 

Quantity 
in  grms. 

O.OOI. 

Small. 

20. 

25-40. 

Na2S04.. 
Na2CO3... 
NaCl  
NaBr  
Nal  
K2SO4.... 
K2CO3.... 
KC1       . 

884 

851 
792 

733 
603 
1052 
880 
762 

883 
86  1 

813 
761 

695 
1059 
893 
800 

880 

820 

765 
650 
1050 
880 
79° 

897 

853 
810 
748 

1074 
894 
778 

KBr.. 

777 

746 

75° 

740 

KI 

614 

723 

705 

680 

Li2SO4...  . 
Li2CO3  .  .  . 
CaCl2  
SrCl2  
BaClj.... 

815 
618 
710 
796 

844 

802 

854 
916 

780 
960 

859 
734 

paraffin,  is  smeared  onto  the  metal  to  be  heated,  and  melting  of 
the  paste  is  readily  recognized.  The  range  covered  by  the  sen- 
tinels and  pastes  is  to  1070°  C. 


Fig.  129.     Sentinel  Pyrometers. 

Any  method  based  on  the  use  of  fusing  points  alone,  whether 
metals,  alloys,  or  salts,  is  evidently  a  discontinuous  one,  and  has- 
its  main  usefulness  in  processes  where  only  a  maximum  or  mini- 
mum temperature  is  required. 


368  HIGH  TEMPERATURES 

Fusible  Cones.  —  Instead  of  utilizing  the  fusion  of  crystallized 
substances  which  pass  abruptly  from  the  solid  to  the  liquid  state, 
use  may  be  made  of  the  progressive  softening  of  vitreous  matters, 
that  is  to  say,  of  mixtures  containing  an  excess  of  one  of  the  three 
acids,  silicic,  boric,  or  phosphoric.  It  is  necessary  in  this  case  to 
have  a  definite  process  for  defining  a  type  degree  of  softening;  a 
definite  depression  of  a  prism  of  given  size  is  taken.  These 
small  prisms,  formed  of  vitreous  matters,  are  known  under  the 
name  of  fusible  cones. 

This  method  was  first  devised  by  Lauth  and  Vogt,  who  applied 
it  in  the  manufactures  at  Sevres  before  1882.  But  they  did  not 
develop  it  as  far  as  was  possible;  they  were  content  to  construct 
a  small  number  of  fusible  cones  corresponding  to  the  various 
temperatures  employed  in  the  manufacture  of  the  Sevres  porce- 
lain. 

Seger,  director  of  a  research  laboratory  at  the  royal  pottery 
works  of  Berlin,  published,  in  1886,  an  important  memoir  on  this 
question.  He  determined  a  whole  series  of  fusible  cones  known 
as  Seger  cones,  of  intervals  of  about  25°,  including  the  interval  of 
temperature  from  600°  to  1800°.  The  substances  which  enter 
into  the  composition  of  these  cones  are  essentially: 

Pure  quartz  sand; 

Norwegian  feldspar; 

Pure  carbonate  of  lime; 

Zettlitz  kaolin. 

The  composition  of  this  last  is: 

SiO2 46.9 

A12O3 38.6 

FeO3 0.8 

Alkalies i .  i 

Water 12.7 

In  order  to  obtain  very  infusible  cones,  calcined  alumina  is 
added,  and  for  very  fusible  cones  oxide  of  iron,  oxide  of  lead, 
carbonate  of  soda,  and  boric  acid. 

The  shape  of  these  cones  (Fig.  130)  is  that  of  triangular  pyra- 
mids of  15  mm.  on  a  side  and  50  mm.  high.  Under  the  action 
of  heat,  when  softening  begins,  they  at  first  contract  without 


VARIOUS  PYROMETRIC   METHODS 


369 


change  of  form,  then  they  tip,  bending  over,  letting  their  apex 
turn  downwards,  and  finally  flattening  out  completely.  One 
says  that  the  cone  has  fallen, 
or  that  it  has  melted,  when 
it  is  bent  halfway  over,  the 
point  directed  downwards. 

The  fusing  points  of  these 
substances  have  been  deter- 


Fig.  130.     Seger  Cones. 


mined  at  the  Berlin  porcelain 
works  by  comparison  with  the 
Le  Chatelier  thermoelectric  pyrometer,  previously  described. 

The  cones  are  numbered,  for  the  less  fusible,  which  were  first 
adjusted,  from  i  to  38;  this  last,  the  least  fusible,  corresponds 
to  1980°.  The  second  series,  more  fusible,  and  established  later, 
by  Cramer  and  Hecht,  is  numbered  from  01  to  022 ;  this  last  cone, 
the  most  fusible,  corresponds  to  590°. 

If,  instead  of  using  the  cones  of  German  make,  one  wishes  to 
make  them  himself  in  employing  the  same  formulae,  it  is  prudent 
to  make  a  new  calibration.  The  kaolins  and  feldspars  from 
different  sources  never  have  exactly  the  same  compositions,  and 
very  slight  variations  in  their  amounts  of  contained  alkali  may 
cause  marked  changes  in  the  fusibility,  at  least  for  the  less  fusible 
cones.  It  is  also  well,  on  this  account,  to  compare  the  behavior 
of  new  cones  with  old,  even  from  the  same  maker. 

It  is  to  be  noticed  that  in  a  great  number  of  cones  silica  and 
alumina  are  found  in  the  proportions  A12O3  +  10  SiC>2.  This  is 
for  the  reason  that  this  mixture  is  more  fusible  than  can  be  had 
with  silica  and  alumina  alone.  It  is  the  starting  point  to  obtain 
the  other  cones,  the  less  fusible  by  the  addition  of  alumina,  and 
the  more  fusible  by  the  addition  of  alkaline  bases. 

The  table  on  pages  371  and  372  gives  the  list  of  cones  of  Seger 's 
scale  as  they  were  originally  issued. 

These  cones  may  be  classed  in  a  series  of  groups,  in  each  of 
which  the  compositions  of  different  cones  are  derived  from  that 
of  one  of  them,  generally  the  most  fusible,  by  addition  in  varying 
proportions  or  sometimes  by  substitution  of  another  substance. 


370 


HIGH  TEMPERATURES 


The  cones  28  to  38  are  derived  from  the  cone  27  by  the  addi- 
tion of  increasing  quantities  of  A12C>3. 

The  cones  5  to  28  from  the  cone  5  by  addition  of  increasing 
quantities  of  the  mixture  A12O3  +  10  SiO2. 

The  cones  i  to  5  from  the  cone  i  by  substitution  of  increasing 
quantities  of  alumina  for  the  sesquioxide  of  iron. 

The  cones  oio  to  i  from  the  cone  i  by  the  substitution  of  boric 
acid  for  silica. 


000° 


Fig.  131.    Composition  of  Seger  Cones. 

The  cones  022  to  oil  from  the  cone  022  by  the  addition  of 
increasing  quantities  of  the  mixture  A1203  +  2  Si02. 

Fig.  131  gives  the  graphical  representation  of  these  data;  the 
ordinates  are  temperatures,  and  the  abscissae  are  values  of  x 
from  the  table. 

These  fusible  cones  of  Seger  are  pretty  generally  used  in  the 
ceramic  industry;  they  are  very  convenient  in  all  intermittent 
furnaces  whose  temperature  has  to  increase  constantly  up  to  a 


VARIOUS   PYROMETRIC    METHODS 


371 


•certain  maximum,  at  which  point  the  cooling  off  is  allowed  to 
commence.  It  is  sufficient,  before  firing  up,  to  place  a  certain 
number  of  fusible  cones  opposite  a  draft  hole  closed  by  a  glass, 
through  which  they  may  be  watched.  In  seeing  them  fall  suc- 
cessively, one  knows  at  what  moments  the  furnace  takes  on  a 
series  of  definite  temperatures. 

In  continuous  furnaces,  the  cones  may  be  put  into  the  furnace 
during  the  process,  but  that  is  more  delicate.  It  is  necessary  to 
place  them  on  little  earthenware  supports  that  are  moved  into 
the  desired  part  of  the  furnace  by  an  iron  rod.  When,  on  the 
contrary,  they  are  put  in  place  at  the  start  in  the  cold  furnace, 
they  are  held  in  place  by  a  small  lump  of  clay. 


THE  ORIGINAL  SEGER  CONE  SCALE. 


Nos. 

Dcg' 

Composition. 

X 

Formulae. 

38 

1890 

A1202+I 

S02 

9 

36 

1850 

"     +1  5 

8 

35 

1830 

+2 

34 

1810 

XA12O3 

33 
32 

1790 
1770 

+4 

.    +(i-X)(Al,Ol 
+10  Si02) 

31 

1750 

+5 

30 

1730 

+6 

29 

1710 

+8 

1 

28 

1690 

+  10 

27 

1670 

(  0.3  K20 
|o.7CaO 

}  +2o(Al2O3+io  SiO,) 

0 

26 

1650 

+7.2 

93 

25 

•1630 

" 

+6.6 

24 

1610 

" 

+6 

23 

1590 

" 

+5.4 

22 

1570 

** 

+4-9 

21 

1550 

+4-4 

20 

1530 

" 

+3.9 

, 

19 

1510 

+3-5 

18 

1490 

" 

+3.1 

X(Al2O3+ioSiO2) 

17 

1470 

" 

+2.7 

.  .      Y     /o  3  K2 

3J 

16 

1450 

+2.4 

79 

-HI-  A)  ^0;7Ca 

15 
14 

1430 
1410 

.. 

+2.1 
+1.8 

+o.s(A!2O3+ioSiO2)) 

13 

1390 

41 

+1.6 

12 

1370 

" 

+1.4 

II 

1350 

" 

+  1.2 

58 

10 

1330 

*• 

+  1 

9 

1310 

+0.9 

8 

1290 

* 

+0.8 

7 

1270 

* 

+0.7 

6 

1250 

4 

+0.6 

5 

1230 

4 

+0.5 

0 

4 

1210 

+o.sA!2O3+4SiO2 

i 

3 

2 

1190 
1170 

i 

i 

+l"fiM}+^' 

X  (0.5  Al2O3+4SiOj) 
•     +  (i-X).  (o.5Fe20 
+4SiO2+o.7CaO) 

I 

1150 

i 

+  \o.l  S&j+'ao, 

372 


HIGH  TEMPERATURES 


THE  ORIGINAL  SEGER   CONE    SCALE   (Continued}. 


Nos. 

D«f 

Composition. 

X 

Formulae. 

01 

1130 

1  {  0.7  CaO  }  +  {  0.2  FejOa  J  '  + 

3-95  Si02 
o.osB2O3 

1.05 

02 

IIIO 

i           "         +                        + 

3-9oSiO2 
o.  10  B2O3 

03 
04 
05 

1090 
1070 
1050 

i           "         +                        + 
i           "          +                        + 

3.85  SiO2 
o.  15  B2O3 
3.8oSiO2 
0.20  B2O3 
3.75  Si02 
o.25B203 

1.25 

—  (SiO2-B2O3) 

06 

1030 

i           "          +i           "          + 

3.7oSiO2 
0.30  B2O3 

•+d-x)  (0;7C|oj 

07 

1010 

i           "          +i           "          + 

3.65  SiO2 
o.3S  B203 

+  {o>:2F(L203}+4Si0») 

•  j           ..           ,   (3.6oSiO2 

08 

900 

|o.4oB203 
3.55  SiO2 

09 

970 

i                      +i                      + 

0.45  B203 

010 

on 

950 
920 

'{;i»p£-'  «*>  + 

3.5    SiO2 
0.5    B203 
13.6  SiO2 
[i.oB2O3 

5 
0.62 

012 

890 

I             "          +0.75      "        + 

3-5  SiO2 
i.oB2O3 

013 

860 

I             "          +0.70      "        + 

3-4Si02 
i.oB2O3 

014 

830 

i             "          +0.65      "        + 

3.3Si02 
i  .  o  B2O3 

015 

016 
017 
018 

800 
770 
740 
710 

i             "          +0.60     "        + 
i             "          +0.55      "        + 
i             "          +0.50     "        + 
i             "          +0.40     "        + 

3.2Si02 
i  .  o  B2O3 
3.iSi02 
i.oB2O3 
3.oSiO2 
i.oB2O3 
2.8SiO2 
i.oBsO, 

0.57 

X(2  Si02+Al203)N 
'         |  2  Si02  iS 

+  \  iB2o3|; 

019 

680 

i             "          +0.30     "        + 

i!oB2O23 

020 

650 

i             "          +0.20      "        +  |?'J§!$. 

021 
022 

620 
590 

I                 "             +0.10       "           + 

I        "                      + 

2.2SiO2 

i  .  o  B2O3 
2.0  SiO2 
i.oB2O3 

0 

Recent  investigations  on  Seger  cones,  in  view  of  their  im- 
provement, have  been  carried  out  mainly  by  the  staff  of  the 
Laboratorium  fur  Tonindustrie,  and  at  the  Reichsanstalt,  and 
consequently  there  have  been  changes  in  their  composition,  melt- 
ing point,  and  numbering.  The  improvements  have  been  mainly 
in  increasing  the  sharpness  of  melting  points,  elimination  in  so 
far  as  possible  of  the  lag  due  to  rate  of  heating,  and  finding 
components  that  are  uninfluenced  by  the  usual  ceramic  furnace 
atmosphere.  This  has  resulted  in  the  elimination  of  lead 
and  iron  compounds.  Cones  Nos.  21  to  25  have  been  dropped, 
as  their  melting  points  were  too  close  together;  and  four  new 


VARIOUS  PYROMETRIC   METHODS 


373 


cones,  Nos.   39  to  42,  the  most  refractory  of  all,  have  been 
added. 

In  the  following  table  are  given  the  melting  points  of  the 
cones  according  to  the  Tonindustrie  Zeitung  circulars  for  1910, 
together  with  their  compositions  when  the  latter  differ  from 
the  original  series. 

1910   SCALE  OF  SEGER  CONES. 


42 

2OOO 

A1203 

4i 

1960 

Al2O3o.i3  SiO2 

40 

I92O 

Al203o.33Si02 

39 

1880 

Al2O3o.66  SiO2 

3» 

1850 

No.              Deg.  C. 

No.                Deg.  C. 

37 

1825 

28                 1630 

14                   1410 

36 

1790 

27                 1610 

13                   1380 

35 

1770 

26                 1580 

12                       1350 

34 

1750 

20                 1530 

II                       1320 

33 

1730 

19                 1520 

10                1300 

32 

I7IO 

18                 1500 

9                 1280 

1690 

17                 1480 

8                1250 

30 

1670 

16                 1460 

7                1230 

29 

1650 

15                H35 

Composition  • 

No. 
6a 

Deg.  C. 

I2OO 

0.013  Na2O   1 
o.288K20       Io6o,  A1( 

D     (6.8oiSiO2 

o.68SCaO      f  °-°93  A12< 

3  \  0.026  B2Os 

0.014  MgO    J 

0.028  Na2O  ^ 

$a 

1180 

0.274  K2O      1              A* 

.    j6.565SiO2 

o  666  CaO      f  °  ' 

J3   1  0.056  B2O» 

0.032  MgO    j 

0.043  Na2O  1 

4& 

1160 

0.260  K2O      1              AI 

-,     (6.  339  Si  O2 

0.649  CaO      rO.676Al2t 

J>   I  0.086  B2O» 

0^048  MgO    J 

0.059  Na2O  1 

3a 

1140 

0.244  K2O      !       6£    Aj  < 

-^     (6.  083  Si  O2 

o  630  CaO      |           /2 

3    /  o.  119  B2Oa 

0.067  MgO    J 

0.085  Na2O  1 

2a 

II2O 

0.220  K2O         1          A    2   AI  ( 

-,    ]5.687SiO2 

o  599  CaO      f                 2 

3   (  0.170  B2OS 

0^096  MgO    J 

o  .  109  Na2O  I 

la 

1  100 

0.198  K2O      !       /•       AI  i 

-^     »5.32oSiO2 

PflO        r°-°39'"-12' 

J*   (0.217  B2Oa 

0.122  MgO 


374  HIGH  TEMPERATURES 

1910  SCALE  OF  SEGER   CONES   (Continued). 

No.      Deg.  C. 
oia     1080 


02  a     1060 


03  a     1040 


1020 


1000 


o6a      980 


07 a      960 


o8a      940 


920 


oioa      900 


ona      880 


OI2E        855 


o.i34  Na2O 
o.  174  K2O 
0.541  CaO 
0.151  MgO 

>  0.625  A12O3 

(  4.931  SiO2 
(0.268  B2O3 

0.157  Na2O 
0.153  K2O 
0.513  CaO 
0.177  MgO 

|-o.6ii  A12O3 

(4-572  SiO2 
(  0.314  B2O3 

0.182  Na2O 
0.130  K2O 
0.484  CaO 
o  .  204  MgO 

Lo.598Al2O3 

(4.199  SiO2 
1  0.363  B203 

0.204  Na2O 
o.  109  K2O 
0.458  CaO 
0.229  MgO 

I  0.586  A12O3 

J3.86oSiO2 
(0.407  B2O3 

0.229  Na2O 
0.086  K2O 
0.428  CaO 
0.257  MgO 

>  0.571  A12O3 

(3.467810., 
1  0.457  B203 

0.247  Na2O 
0.069  K2O 
0.407  CaO 
0.277  MgO    , 

>  0.561  A12O3 

1 

(3.197  Si02 
(0.493  BaOa 

0.261  Na2O  * 
0.055  K2O 
0.391  CaO 
0.293  MgO    > 

^  0.554  A12O3 

(2.984  SiO2 
(0.521  B2O3 

0.279  Na2O   ] 
o.o38K2O      ! 
0.369  CaO      j 
0.314  MgO    J 

^0.543  A12O3 

(2.691  S1O2 

0.336  Na2O   ^ 
0.018  K2O      1 
0.335  CaO 
0.311  MgO    J 

^0.468  A12O3 

(3.o87SiO2 
(0.671  B2O3 

0.338  Na2O  I 
o.on  K2O 
0.338  CaO 
0-313  MgO    j 

-0.423  A12O3 

(2.626  SiO2 
(o.67SB2O3 

0.349  Na2O    ) 
0.340  CaO 
0.311  MgO     ) 

0.4  A12O3 

(  2.38S1O2 
(0.68  B2O3 

0-345  Na2O     ] 
0.341  CaO 
0.314  MgO     ) 

0.365  A1203 

| 

{l:SlSi 

VARIOUS   PYROMETRIC   METHODS  375 

1910  SCALE   OF   SEGER    CONES    (Continued). 


No.  Deg.  C.      f  -  —  Composition. 

0.343  Na2O 

OI3a      83S  ° 


OI4a      8l5 


oisa     79° 


016  750      Bf.*  0.31  A1203  I  I'61 


017  730  Bf.  0.2  Al2 

018  710  Bf.  o.  13  A1203  |  I  •  26  | 

019  690  Bf  .  o  .08  A1203  |  I  •  l6 

020  670  Bf.  0.04  A12O3        < 


021  650       Bf.    o.o2Al2O3j*    >4B2o3 

022  600 


0.50  Na2O 
Bf=     o.2sCaO 
0.25  MgO 


It  would  seem  to  be  well  to  replace,  in  so  far  as  possible,  these 
cones  by  pure  compounds  and  eutectics  having  definite  melting 
points,  as  the  softening  temperatures  of  the  former  are  influenced 
considerably,  in  some  cases  100°  C.  or  more,  by  the  rate  of  heat- 
ing, as  has  been  remarked  by  several  investigators.  Dr.  Kanolt 
of  the  Bureau  of  Standards  has  carried  out  a  series  of  measure- 
ments on  some  of  the"  Standard  Pyrometric  Cones  "  of  Professor 
Orton  of  the  Ohio  State  University,  corresponding  to  Nos.  25  to 
36  of  the  Seger  series,  as  well  as  on  this  Seger  series.  His  work 
shows  that  heating  the  cones  as  rapidly  as  5°  C.  per  minute, 
when  near  their  softening  temperatures,  will  give  too  high 
values  for  these  temperatures.  The  rate  of  heating  in  the 
experimental  or  calibrating  furnace  must  be  reduced  to  more 


376  HIGH  TEMPERATURES 

nearly  that  which  obtains  in  kiln  practice,  in  order  to  get  a  fair 
calibration  of  the  cones  for  use  in  the  ceramic  industries. 

The  Seger  and  Orton  cones  were  found  to  agree  very  closely 
in  their  behavior.  With  slow  heating,  the  cones  of  the  series 
25-36  were  found  to  soften  at  temperatures  lower  by  40°  to  70°  C. 
than  indicated  in  the  table  of  1910,  page  373,  agreeing  closely  in 
this  with  the  results  found  by  Heraeus.  Melting  in  air  or  in 
vacuo  gave  the  same  results.  Kanolt's  measurements  were  made 
with  an  optical  pyrometer  whose  scale  is  represented  by  Au  = 
1064°,  Pd  =  1550°,  Pt  =  1755°  C. 

At  the  Reichsanstalt,  Hoffmann  and  Meissner  find  (1911)  sim- 
ilar differences  between  softening  temperatures  in  ceramic  kilns 
for  a  time  of  heating  of  about  sixty  hours  and  in  the  electric 
furnace. 

SOFTENING  TEMPERATURES  OF  SEGER  CONES. 

Cone  In  electric  In  ceramic         rvffWo 

number.  furnace.  kiln.  Qce' 

6 

8 

9 
10 

13 
14 
16 

17 

On  the  whole,  it  may  be  said  that  the  Seger  cone  series  give 
reliable  relative  temperatures  to  about  25°  C.  at  the  higher  tem- 
peratures for  any  given  method  of  procedure,  but  too  much 
reliance  should  not  be  placed  on  the  numerical  values  of  the 
temperatures  apparently  measured. 

WiborgWs  Thermophones.  —  Another  cheap,  discontinuous  py- 
roscope  has  been  put  on  the  market  by  Wiborgh.  His  thermo- 
phones  are  refractory  earth  cylinders  2.5  cm.  long  and  2  cm.  in 
diameter,  containing  an  explosive.  A  thermophone  is  quickly 
deposited  in  the  region  whose  temperature  is  sought,  and  the 
time  noted  to  the  fifth  of  a  second  until  the  cylinder  bursts.  A 
table  then  gives  the  temperature.  Very  concordant  results  are 


1225 

1160 

65 

1260 

1200 

>6o 

1285 

1180 

105 

1305 

<I200 

105 

1335 

1225 

no 

1345 

1235 

no 

1395 

1315 

80 

1415 

1375 

40 

1460 

1405 

55 

1480 

1410 

70 

VARIOUS   PYROMETRIC   METHODS 


377 


obtained  if  the  thermophones  are  kept  dry,  different  cylinders 
of  the  same  set  agreeing  to  one-fifth  second,  or  20°  C.  at  1000°  C. 
Dilution  Pyrometers.  —  If  a  current  of  liquid  or  gas  is  kept 
flowing  through  a  heated  space,  it  is  evidently  possible  to  estimate 
the  temperature  of  the  latter  by  observing  the  inlet  and  outlet 
temperatures  of  the  fluid.  Carnelly  and  Burton  constructed 
such  a  pyrometer,  using  water  flowing  at  constant  head  from  a 
tank'  kept  at  constant  temperature.  The  graduation  of  such  a 
pyrometer  is  purely  empirical,  and  may  be  effected,  for  a  given 


Fig.  132.     Hot-blast  Pyrometer. 

heat  and  temperature  of  supply,  by  taking  the  inlet  and  outlet 
temperatures  for  three  or  more  known  temperatures  of  the  fur- 
nace. For  every  different  head  and  temperature  of  source  the 
graduation  will  be  different.  Such  a  pyrometer  evidently  re- 
quires a  somewhat  cumbersome,  permanent  installation,  and  has 
the  further  disadvantages  of  not  being  direct-reading  and  having 
its  indications  change  with  difficultly  controllable  factors. 

For  determining  hot-blast  temperatures  air-dilution  pyrom- 
eters have  been  used,  air  from  the  outside  entering  the  blast, 
mixing  with  it,  and  the  temperature  of  the  outcoming  mixture 


378  HIGH  TEMPERATURES 

being  taken  with  a  mercury  thermometer,  and  then  the  tempera- 
ture of  the  blast  computed  from  an  empirical  calibration.  But 
very  uncertain  results  can  be  obtained  in  this  way,  as  they  will 
depend  on  the  speed  of  the  blast,  the  size  of  openings,  and  the 
temperature  of  the  diluting  air.  Such  a  pyrometer  is  illustrated 
in  Fig.  132. 

Pyrometers,  such  as  Carnelly's,  have  also  been  based  on  the 
circulation  of  a  water  stream  whose  inlet  and  outlet  temperatures 
could  be  taken. 

Transpiration  Pyrometers.  —  Various  attempts  have  been 
made  to  construct  pyrometers  based  on  the  variation  of  the 
viscosity  of  gases  with  temperature,  and  this  subject  has  been 
thoroughly  studied  by  Holman,  Barus,  and  Callendar;  but,  owing 
to  the  complexity  of  the  viscosity  temperature  relation  for  small 
tubes,  no  simple  pyrometer  based  on  this  relation  alone,  not 
requiring  an  arbitrary  calibration,  has  been  devised.  This 
method  may  perhaps  serve,  as  first  suggested  by  Barus,  as  an 
independent  one  for  extending  the  temperature  scale  beyond  the 
region  reached  by  other  forms  of  gas  thermometer. 

Job  has  shown  that  if  a  short  piece  of  platinum  wire  be  inserted 
in  the  end  of  a  porcelain  tube  of  less  than  i  mm.  diameter  and  a 
constant  current  of  gas,  as  from  an  electrolytic  cell  or  blower,  be 
passed  through  this  capillary,  the  back  pressure  developed  will 
be  proportional  to  the  temperature,  or  T  =  k(H  —  ho),  where 
H  is  given  by  a  manometer  inserted  between  the  cell  or  blower 
and  the  porcelain  capillary,  and  ho  is  the  initial  pressure.  This 
simple  relation  holds  very  exactly  up  to  temperatures  as  high  as 
1500°  C.,  and  the  method  may  be  made  very  sensitive  by  a  proper 
choice  of  manometer  liquid  and  initial  pressure  h0.  The  indi- 
cations, however,  vary  with  the  depth  of  immersion  of  the 
capillary,  and  they  depend  not  alone  upon  the  viscosity  of 
the  gas,  but  also  upon  the  relative  expansion  of  platinum  and 
porcelain. 

A  pyrometer  depending  upon  the  change  in  pressure  produced 
in  a  current  of  gas  or  vapor  passing  through  a  small  orifice  A 
(Fig.  133),  at  high  temperature  has  been  developed  by  Uhling  and 


VARIOUS   PYROMETRIC   METHODS 


379 


Steinbart,  using  a  steam-jacket  aspirator  D  to  produce  a  steady 
flow.  It  is  essential  that  the  air  pass  through  the  apertures  B 
and  A  B.t  SL  constant  pressure,  as  measured  by  Q,  that  all  the 
air  become  heated  to  a  uniform  temperature  at  A,  that  the 
apertures,  which  are  but  pinholes,  remain  perfectly  clear,  that 
the  temperature  of  the  colder  aperture  B  remain  constant,  and 
that  there  are  no  leaks.  The  hot  end  CA  is  inclosed  within  a 
tube  of  platinum  or  nickel  and  the  air  is  filtered  before  passing 
into  A. 


Fig.  133.     Uhling-Steinbart  Pyrometer. 

Although  simple  in  principle,  the  apparatus  as  constructed  is 
very  complicated  and  costly.  It  is  made  direct-reading  and  also 
recording.  The  calibration  is  empirical  and  the  apparatus  is  so 
constructed  that  temperatures  are  read  off  the  water-manometer 
column,  P.  The  elaborateness  of  construction  of  such  a  pressure 
apparatus  renders  it  liable  to  deteriorate  with  time  and  use,  and 
it  requires  a  source  of  steam  for  its  operation. 

In  Threw's  pyrometer,  air  under  constant  pressure  is  forced 
through  a  coiled  tube  in  the  heated  region,  and  the  back  pressure 
developed  between  a  hot  and  cold  orifice  is  measured  on  a  water 
column  in  much  the  same  way  as  in  Uhling's  apparatus.  Both 
these  instruments  have  been  used  considerably  in  blast-furnace 
practice. 


380  j      HIGH  TEMPERATURES 

Vapor-pressure  Pyrometers.  —  Use  is  made  of  the  fact  that 
the  pressure  of  a  saturated  vapor,  or  one  in  the  presence  of  its 
liquid,  depends  only  on  the  temperature  of  the  vapor,  and  is 
independent  of  its  volume.  Readings  of  such  a  pyrometer  may 
reduce  to  those  of  a  pressure  gauge.  There  is  an  apparent  ad- 
vantage over  the  gas  thermometer,  in  that  the  volume  of  the 
containing  vessel  plays  no  part.  This  vessel  must  be  gas-tight, 
however,  in  both  types.  For  relatively  low  temperatures,  where 
ether  or  water  may  be  used,  to  350°  C.  for  water,  several  indus- 
trial forms  using  this  principle  have  been  developed,  that  have 
given  satisfaction  in  special  installations,  notably  the  thalpo- 
tassimeter  of  Schaffer  and  Budenberg  and  the  instruments  of 
Fournier. 

The  difficulty  of  rendering  them  gas-tight  and  permanent 
when  mercury  or  other  substance  suitable  for  higher  tempera- 
tures is  used  appears  to  be  a  serious  obstacle  to  the  general 
introduction  of  this  type  of  instrument  as  a  pyrometer. 

Other  Pyrometric  Methods.  —  We  have  by  no  means  ex- 
hausted the  list  of  methods  for  measuring  high  temperatures  that 
have  been  suggested  or  tried.  Without  dwelling  on  any  of  them, 
we  may  mention  a  few  that  may  possibly  be  of  service  in  particu- 
lar cases.  The  variation  of  boiling  point  with  pressure  of  such 
substances  as  naphthaline,  benzophenone,  and  sulphur,  first 
studied  in  detail  by  Crafts,  in  1882,  will  give  a  continuous  tem- 
perature scale  of  very  great  range,  although  a  relatively  com- 
plicated pressure  apparatus  is  necessary.  Again,  the  velocity 
of  sound  in  any  medium  is  a  function  of  its  temperature;  and,  as 
early  as  1837,  a  method  of  temperature  measurement  using  this 
principle  was  devised  by  Cagniard-Latour  with  dry  air  as  the 
medium.  Other  phenomena  of  less  promise  which  have  been 
made  use  of  or  suggested,  are:  heat  conduction,  rotary  polari- 
zation, magnetic  moment,  dissociation,  conductivity  of  gases 
and  vapors,  the  corpuscular  emission  in  vacuo  from  current- 
.bearing  metals,  and  an  application  of  Clapeyron's  Equation. 


CHAPTER  X. 
RECORDING  PYROMETERS. 

AMONG  the  different  methods  for  the  measurement  of  high 
temperatures,  some  of  them  may  be  made  continuously  record- 
ing. This  is  as  useful  for  industrial  applications  as  for  scientific 
investigations.  In  research  laboratories  one  endeavors  as  much 
as  possible  to  take  observations  automatically,  escaping  the 
influence  either  of  preconceived  ideas  or  of  carelessness  of  the 
observer;  in  industrial  works  the  use  of  such  processes  gives 
continuous  control  over  the  work  of  the  artisans,  such  as  the 
presence  of  no  foreman  can  replace. 

In  recent  years,  one  of  the  most  important  practical  advances 
that  have  been  made  in  pyrometry  is  the  development  of  sev- 
eral types  of  simple,  convenient,  and  reliable  instruments  for 
the  registration  of  temperatures  in  industrial  operations.  It 
will  be  impossible  to  describe  them  all  here,  but  we  shall  pass 
in  review  several,  as  well  as  calling  attention  to  the  historical 
development  of  the  subject.  Forms  of  temperature-recording 
apparatus,  suitable  for  laboratory  investigations  of  problems 
involving  temperature  changes,  but  too  complicated  for  any  but 
the  most  elaborate  technical  installations,  have  been  in  existence 
for  a  good  many  years,  and  the  recent  introduction  of  more  simple 
apparatus  has  greatly  stimulated  technical  research  as  well  as 
afforded  means  for  the  exact  control  of  a  great  many  industrial 
operations  that  were  heretofore  left  to  chance. 

Forms  of  Temperature  Records.  —  There  are  several  ways  in 
which  the  change  of  temperature  with  time  may  be  recorded,  and 
the  method  adopted  will  depend  upon  the  problem  in  hand. 
The  simplest  and  the  one  of  most  universal  application  and 
general  utility  both  in  the  works  and  laboratory  is  the  time- 
temperature  curve;  that  is,  the  time  appears  as  one  coordinate, 

381 


382  HIGH  TEMPERATURES 

and  the  temperature,  or  some  quantity  proportional  to  it,  as  the 
other  on  the  record  sheet.  Most  temperature  recorders  are  con- 
structed on  this  basis.  For  a  given  temperature  interval,  there 
is  evidently  a  limiting  sensibility  beyond  which  any  such  recorder 
cannot  go  without  unduly  increasing  the  size  of  the  record  sheet 
unless  it  is  operated  in  steps,  when  of  course  the  apparatus  be- 
comes only  semi-recording  and  requires  the  occasional  interven- 
tion of  the  operator. 

In  certain  investigations  in  fhe  laboratory  it  is  of  interest  to 
have  the  rate  of  change  of  temperature  in  terms  of  the  tem- 
perature, or  the  temperature-rate  curve.  Thus,  in  the  study  of 
definite  phenomena,  such  as  fusion  and  allotropic  transforma- 
tions, and  in  order  to  recognize  their  occurrence,  use  is  ordinarily 
made  of  the  accompanying  absorption  or  liberation  of  heat, 
which  is  manifested  by  a  variation  in  the  rate  of  heating  or  cool- 
ing. Methods  of  recording  the  temperature-rate  curve  have 
been  devised  by  Le  Chatelier  and  by  Dejean. 

In  the  two  preceding  methods,  any  accidental  variation  in  the 
temperature  of  the  furnace,  due  to  drafts  or  other  outside  causes, 
or  to  change  in  rate  of  heating  or  cooling,  will  be  recorded.  This 
is  most  desirable  in  those  cases  in  which  the  changes  in  tem- 
perature of  the  furnace  or  its  contents  are  wanted.  But  in  those 
cases  in  which  the  transformations  going  on  within  the  substance 
itself  within  the  furnace  are  wanted,  as,  for  example,  when  taking 
the  cooling  curve  of  a  sample  of  steel,  the  accidental  fluctuations 
in  the  heat  content  not  inherent  to  the  sample  must  be  elimi- 
nated in  exact  work.  This  may  be  done,  as  shown  by  the  late 
Sir  Roberts-Austen,  by  taking  the  differential-temperature  curve,  or 
recording  the  temperature  of  the  sample  in  terms  of  the  successive 
differences  in  temperature  between  the  sample  under  observation 
and  another  body,  called  the  neutral,  possessing  no  transformation 
points,  and  placed  within  the  furnace  close  to  the  sample. 

Although  there  are  other  methods  of  reading  and  interpreting 
temperature  curves,  the  above  are  the  only  ones  that  have  here- 
tofore been  made  self-recording.  We  shall  describe  types  of 
apparatus  using  all  three  methods. 


RECORDING  PYROMETERS 


383 


Types  of  Cooling  Curves.  —  It  is  of  interest  to  be  able  to  recog- 
nize the  appearance  of  a  transformation  region  as  denned,  for 
example,  on  cooling  by  the  various  methods  above  mentioned. 
In  Fig.  134  are  illustrated  these  various  forms  of  temperature 
curves,  in  which  6  =  temperature  and  /  =  time  for  the  three 
following  cases:  in  the  first  line,  the  heat  evolution  balances  the 
radiation  and  other  losses  during  the  transformation  or  the  tern- 


Temperature  —Time 


Differential 


Temperature  -Rate 


Inverse  Rate 


dO/dt 


TYPES  OF  COOLING  CURVES 

Fig.  134.     Types  of  Cooling  Curves. 

perature  remains  constant,  as  in  freezing  of  a  pure  substance  well 
inclosed;  in  the  second  line,  the  most  common,  the  transforma- 
tion takes  place  over  a  definite  temperature  interval  or  there  is 
imperfect  heat  insulation;  and  in  the  third,  there  is  recalescence, 
or  the  reaction  or  transformation  evolves  heat  so  rapidly  as  to 
cause  a  rise  in  temperature.  The  nomenclature  is  the  same  for 
all  the  curves,  and  they  may  be  compared  by  noting  the  cor- 
responding letters. 


384  HIGH  TEMPERATURES 

We  have  included  also  the  inverse-rate  curve  of  Osmond,  ob- 
tained by  noting  the  time  intervals  necessary  to  cool  equal  decre- 
ments of  temperature  plotted  in  terms  of  temperature.  Although 
this  curve  cannot  be  recorded  automatically  without  elaborate 
apparatus,  the  inverse-rate  method  is  one  in  very  common  use 
in  metallographic  practice.  Rosenhain  uses  still  another  form 
of  curve,  the  derived-differential,  or  the  temperature  difference 
between  sample  and  neutral  for  equal  temperature  decrements 
plotted  in  terms  of  temperature.  The  observations  are  taken 
as  if  for  the  differential  curve. 

On  the  whole,  the  most  complete,  satisfactory,  conveniently 
recorded,  and  readily  interpreted  results  are  obtained  over  long 
temperature  intervals  by  the  simultaneous  recording  or  observ- 
ing of  the  time-temperature  and  differential  curves;  or,  as  we 
shall  see,  it  is  possible  to  combine  these  two  into  a  single  curve 
giving  differences  between  sample  and  neutral  in  terms  of  tem- 
perature. The  time  may  also  be  indicated  if  desired  in  this 
latter  case  by  a  suitable  interrupter. 

The  inverse-rate  curve  may  be  taken  with  great  precision  on  a 
chronograph  in  connection  with  a  potentiometer,  the  observer 
pressing  a  key  at  equal  steps  on  the  dials,  and  this  method  may 
be  made  as  accurate  as  desired  if  automatic  control  of  the  fur- 
nace is  provided.  The  form  of  the  curve  obtained  is  exactly  the 
same  as  that  with  the  derived  differential  method.  These  two 
methods  may  be  applied  simultaneously  to  advantage  on  the 
same  sample,  as  is  the  practise  at  the  Bureau  of  Standards. 

Methods  of  Recording.  —  The  recording  may  be  effected  either 
photographically,  or  by  some  electrical  or  mechanical  means  for 
which  we  may  use  the  term  "  autographic  ".  The  latter  possesses 
the  advantage  that  the  experimenter  may  watch  any  part  of  the 
record,  and  can  therefore  control  the  operation  and  at  any 
moment  vary  the  conditions  affecting  the  experiment  or  process; 
whereas,  with  the  photographic  apparatus,  it  is  usually  necessary 
to  wait  until  the  plate  is  developed  to  see  what  has  happened. 
The  manipulation  by  the  photographic  method  is  usually  also 
more  delicate  and  time-consuming  and  the  adjustment  less  sure, 


RECORDING  PYROMETERS  385 

and  the  record  often  requires  further  graphical  interpretation, 
for  all  of  which  reasons  the  photographic  method  is  not  adapted 
for  most  industrial  needs.  The  autographic  method  is  in  general 
not  adapted  for  interpreting  phenomena  taking  place  within  an 
interval  of  a  few  seconds,  so  that  for  very  rapid  temperature 
changes  it  is  usually  necessary  to  employ  the  photographic 
method.  An  autographic  recorder  is  usually  the  less  readily 
adjusted  for  very  variable  rates,  and  most  types  of  such  appa- 
ratus are  limited  to  one  or  two  speeds.  This  is  not  a  serious 
inconvenience,  however,  except  in  case  of  the  use  of  the  same 
instrument  for  very  diverse  purposes. 

The  following  pyrometers  have  been  made  recording : 

The  constant- volume  gas  thermometer; 

The  thermoelectric  pyrometer; 

The  electrical  resistance  pyrometer; 

The  total-radiation  pyrometer; 

The  transpiration  pyrometer. 

The  optical  pyrometer  of  the  Morse  type  could  be  made  semi- 
recording,  but  the  other  optical  and  discontinuous  types  of  py- 
rometer could  be  made  recording  only  with  very  great  difficulty. 

Recording  Gas  Pyrometer.  —  The  transformation  of  the  gas 
pyrometer  into  a  recording  instrument  is  extremely  simple  and 
has  been  long  since  effected.  It  suffices  to  join  permanently  the 
tube  from  the  porcelain  bulb  to  a  registering  manometer  to  realize 
a  recording  pyrometer  theoretically  perfect.  But  practically 
these  instruments  possess  many  disadvantages  that  have  pre- 
vented their  introduction  generally,  and  they  have  been  replaced 
for  the  most  part  by  other  types  more  suitable  both  for  laboratory 
and  technical  plants. 

Above  1000°  the  permeability  of  the  porcelain  for  water  vapor 
is  sufficient  to  soon  render  them  useless.  Investigations  made 
by  the  Paris  Gas  Company  have  shown  that  in  furnaces  heated 
to  1100°  the  penetration  of  water  vapor  is  sufficiently  rapid  so 
that  in  a  few  days  liquid  water  collects  in  the  cold  parts  of  the 
apparatus. 

Absolute  impermeability  of  the  apparatus,  which  is  quite  in- 


386  HIGH  TEMPERATURES 

dispensable,  since  its  operation  supposes  the  invariability  of  the 
gaseous  mass,  is  very  difficult  to  obtain.  Frequently  the  glazing 
of  the  porcelain  has  holes  in  it.  The  numerous  joints  entering 
into  the  registering  apparatus,  and  above  all  the  metallic  parts  of 
the  apparatus,  may  be  the  seats  of  very  small  leakages  difficult 
to  locate. 

The  connection  of  the  metallic  parts  with  the  porcelain  tube  is 
generally  made  with  wax,  always  with  substances  of  organic 
origin,  which,  in  the  vicinity  of  industrial  apparatus,  generally 
bulky  and  thick- walled,  cannot  be  protected  against  radiation 
save  by  a  water  jacket.  This  is  a  serious  inconvenience. 

In  laboratory  apparatus  of  small  size  the  protection  of  the 
joint  is  easier,  but  then  the  large  dimensions  of  the  bulb,  as  has 
been  indicated,  are  a  serious  disadvantage.  One  cannot,  in  a 
small  furnace,  find  a  large  volume  whose  temperature  is  uniform. 

Another  most  serious  disadvantage  of  the  recording  gas  pyrom- 
eter is  the  difficulty  of  its  calibration.  Already  with  the  mercury 
manometer  the  dead  space  is  a  source  of  complications.  How- 
ever, this  may  be  measured  and  allowed  for.  With  the  register- 
ing manometer  the  dead  space  is  much  greater,  and  besides 
variable  with  the  deformation  of  the  elastic  tube.  Thus  the 
calibration  can  be  made  only  empirically,  employing  baths  of 
fixed  fusing  or  boiling  points,  an  operation  almost  always  im- 
possible of  realization  with  an  apparatus  of  very  fragile  porcelain, 
or  by  using  a  large  tube  furnace  whose  temperatures  are  given 
by  another  type  of  pyrometer  calibrated  at  the  requisite  fixed 
points.  The  recording  gas  thermometer  is  therefore  of  little 
practical  interest. 

Electrical  Resistance  Recording  Pyrometer.  —  There  have 
been  several  satisfactory  solutions  of  the  problem  of  rendering 
the  resistance  pyrometer  self-registering. 

Callendar^s  Slidewire  Recorder.  —  In  order  to  render  his  py- 
rometer recording  (Figs.  135  and  137),  Callendar  employs  the 
following  very  simple  device:  Two  of  the  branches  of  a  Wheat- 
stone  bridge  used  to  measure  the  resistance  of  the  heated  coil  are 
made  of  a  single  wire,  on  which  slides  a  rider  to  which  i§  brought 


RECORDING  PYROMETERS  387 

one  of  the  galvanometer  leads.  To  each  position  of  the  rider, 
when  the  galvanometer  is  at  zero,  corresponds  a  resistance,  and 
consequently  a  definite  temperature  of  the  coil.  The  position 
of  the  rider  may  be  easily  registered  by  attaching  to  it  a  pen 


Fig.  135.     Calendar's  Recorder. 

writing  on  a  sheet  of  paper  which  moves  perpendicularly  to  the 
length  of  the  wire.  In  order  to  have  the  curve  thus  obtained 
correspond  to  that  of  temperatures,  it  suffices  that  the  position 
of  the  rider  be  at  each  instant  adjusted  so  as  to  keep  the  gal- 
vanometer at  zero. 


388 


HIGH   TEMPERATURES 


This  result  is  obtained  by  means  of  a  clock  movement  con- 
trolled by  a  relay  that  the  galvanometer  works  in  one  direction 

or  the  other,  according  to 
the  direction  of  the  deflec- 
tion that  it  tends  to  take 
on  from  the  zero  point. 
It  is  a  movable-coil  gal- 
vanometer whose  needle 
carries  an  arm  which,  mak- 
ing contact,  causes  a  cur- 
rent to  pass.  The  curve 
traced  by  this  instrument 
is  in  rectangular  coordi- 
nates,  which  is  of  practical 
convenience  in  reading  off 
temperatures. 

Fig.  136  gives  an  example 
of  a  curve  recorded  by  this 
apparatus,  showing  the  ef- 
feet  on  the  temperature 
of  an  annealing  oven  by 
firing  by  an  old  hand  and 
by  a  new  one.  It  will  be 
noted  that  a  continuous 
pen  record  is  obtained. 

This  recorder  possesses 
an  interesting  detail  which 
assures  good  working  and 
which  could  well  be  adopted 
in  other  similar  cases.  The 
pointer  of  the  galvanometer 
needle  does  not  hit  against 
a  fixed  conductor,  to  which 
it  would  stick  on  account  of  heating  by  the  passage  of  the 
current  and  especially  the  extra  current  at  break.  This  con- 
ductor consists  of  the  metallic  circumference  of  a  wheel  which 


RECORDING  PYROMETERS  389 

is  given  a  slow,  constant  rotary  motion,  rendering  all  adherence 
impossible.  This  artifice  renders  possible  working  the  relays  by 
means  of  a  sensitive  galvanometer,  which  would  not  otherwise 
be  realizable. 

This  registering  apparatus  is  necessarily  very  costly,  but  until 
recently  was  the  only  sensitive  one  generally  available  with 
which  a  record  of  high  temperatures  could  be  obtained  by  purely 
mechanical  means  without  the  intervention  of  photography. 
This  apparatus  labors  further  under  the  disadvantages  inherent 
to  a  somewhat  complicated  mechanism,  requiring  the  inter- 
vention of  skilled  labor  to  adjust  it;  nevertheless,  it  has  proved 
of  extreme  usefulness  in  many  technical  operations  requiring 
exact  temperature  control,  especially  in  large  works. 

For  use  in  the  laboratory,  it  is  to  be  noted  that  some  of  the 
extreme  sensitiveness  of  the  resistance  method  of  measuring 
temperatures  is  necessarily  lost  by  rendering  it  recording;  but 
such  loss  is  not  serious  enough  to  prevent  this,  the  most  sensitive 
method  of  recording,  to  be  of  great  use  in  many  laboratory 
investigations.  A  sensibility  of  i  in  5000  may  easily  be  main- 
tained, and  with  care  considerably  higher  sensibility.  The 
relatively  large  volume  of  the  bulb,  or  thermometric  coil,  is 
sometimes  a  disadvantage,  as  in  taking  cooling  curves  of  steel 
specimens.  Callendar  calls  attention  also  to  the  following 
points :  The  scale  of  the  instrument  is  uniform  and  independent 
of  the  E.M.F.  of  the  battery,  being  determined  by  the  resistance 
of  the  bridge  wire  in  relation  to  that  of  the  thermometer,  and  these 
may  be  constructed  to  give  any  desired  temperature  range.  It 
is  a  great  advantage  in  practice  that  the  scale  never  requires 
adjustment,  but  is  always  correct  to  about  i  in  1000,  provided 
the  bridge  wire  is  correct  and  uniform. 

For  ordinary  work,  thermometers  are  generally  provided  with 
an  "ice  bobbin"  or  balancing  coil,  for  adjusting  the  resistance 
of  the  thermometer  at  o°  C.  The  ice  bobbin  for  each  thermom- 
eter is  connected  to  its  appropriate  terminals  when  the  ther- 
mometer to  which  it  belongs  is  in  use.  If  the  thermometer  is 
required  to  cover  an  extensive  range  of  temperature,  and  it  is 


390  HIGH  TEMPERATURES 

desired  to  keep  the  sensitiveness  very  great,  a  series  of  auxiliary 
resistances  or  "zero  coils,"  generally  ranging  from  o  to  20  ohms, 
may  be  provided,  which  enables  the  range  to  be  extended  to 
twenty  times  that  of  the  i-ohm  bridge  wire.  With  a  26-ohm 
thermometer  the  range  thus  obtained  would  be  200°  C.,  or 
2000°  C.  with  a  2.6-ohm  pyrometer. 

The  most  important  points  to  test  in  a  slide-wire  recorder  are 
the  adjustment  of  the  zero  of  the  galvanometer  and  the  zero  of 
the  slide  wire.  The  former  is  effected  by  turning  the  torsion 
lead  of  the  suspension  until  the  galvanometer  boom  swings  clear 
of  the  contact  wheel;  and  for  the  latter,  the  slide  wire  itself  may 
be  shifted  with  respect  to  the  record  paper,  the  pyrometer  and 
compensation  terminals  being  short-circuited  and  the  battery 
switched  on. 

This  recorder,  as  well  as  the  others  we  shall  mention,  may  be 
arranged  to  register  any  of  the  component  factors  in  electric 
power,  and  may  therefore  serve  also  as  a  thermoelectric  recorder. 

When  so  used  for  the  measurement  of  temperatures  by  means  of 
thermoelectric  couples  by  the  method  of  opposition,  the  strength 
of  the  currents  available  to  work  the  relays  is  much  more  feeble 
than  in  the  preceding  applications,  so  that  a  great  sensibility 
cannot  be  obtained.  Fig.  136  shows  a  Callendar  recorder  as 
made  by  the  Cambridge  Instrument  Company,  arranged  for 
the  thermoelectric  measurement  of  temperatures  in  connection 
with  an  electric  resistance  furnace.  In  Fig.  137  is  shown  the 
wiring  diagram  for  a  Callendar  recorder  arranged  for  commercial 
work. 

Deflectional  Recorders.  —  The  thread  recorder,  shown  in  Fig. 
152,  or  other  type  of  recording  millivoltmeter  designed  primarily 
for  use  with  thermocouples,  may  also  be  used  to  register  rela- 
tively small  intervals  of  temperature  when  used  as  the  deflect- 
ing galvanometer  of  a  Wheatstone  bridge  for  a  resistance 
thermometer,  provided  the  sensitiveness  of  the  recording  instru- 
ment is  sufficient.  This  is  readily  accomplished,  since  a  high- 
resistance  galvanometer  is  not  required  nor  desired.  In  Fig.  138 
is  shown  such  an  arrangement  as  used  by  Callendar.  The 


RECORDING    PYROMETERS 


391 


^ 

wj 

Release  Magnet     J 

/\  Cut  Out 
—  '\ 

tiU 

Release  Magnet 

Cut  Out  \ 

/     ^ 

V  ,.,. 

p-     !> 

v     Galvanometer  Wire 

n                 *         £  ""                                '  " 

v/WWVWW/WAAWW^A 

| 
1 

1 
I 

C             Bridge  Wire                     I  LJ- 
yVWW\A/>^/WWWWVWNA/WV  

Battery 


Thermometer  Bulb  or 
Resistance  to  be  Measured 


Fig.  137.     Wiring  Diagram,  Callendar  Recorder. 


392 


HIGH  TEMPERATURES 


Siemens  and  Halske  recorder  (Fig.  149)  may  also  be  had  with  a 
very  sensitive  scale,  i.e.,  1.5  millivolt,  for  example,  for  a  galva- 
nometer resistance  of  10.5  ohms. 

Some  form  of  platinum-thermometer  bridge  with  a  slide  wire 
is  required,  and  a  rheostat  capable  of  fine  adjustment.  This 
rheostat  is  connected  in  series  with  a  storage  cell  and  serves  to 
adjust  the  scale  of  the  record.  One  terminal  of  the  recorder  is 
connected  at  G  between  the  ratio  arms  of  the  bridge  BiG  and 
and  the  other  to  the  sliding  contact  on  the  bridge  wire. 


Rh 


Plugs 


I, 


Slide  Wire 


/wwvw 
Pt 

Fig.  138.     Circuits  for  Deflectional  Recorder. 


The  compensator  leads  are  connected  in  series  with  the  balancing 
coils.  With  this  arrangement  the  resistance  S  +  C  is  nearly 
constant,  so  that  if  the  galvanometer  deflection  is  adjusted  to  be 
correct  when  the  thermometer  is  at  o°  C.,  or  any  convenient 
temperature,  the  scale  will  be  very  nearly  correct  when  the  ther- 
mometer is  at  any  other  temperature,  the  plugs  R  being  suitably 
adjusted.  A  much  greater  sensibility  can  be  obtained  in  this 
way  than  with  thermocouples  using  such  a  recorder.  Thus  it 
is  possible  to  get  a  scale  of  50  mm.  to  i°  C.  if  desired.  Such  great 
sensibility,  however,  would  only  be  necessary  in  extremely  deli- 


RECORDING   PYROMETERS  393 

cate  thermostatic  control.  The  scale  of  the  galvanometer  is  not 
strictly  one  of  equal  parts,  and  this  has  to  be  allowed  for  in  exact 
work. 

The  Leeds  and  Northrup  Recorders.  —  This  firm  has  developed 
two  types  of  resistance  recorders  suitable  for  temperature  meas- 
urements, one  having  an  electromagnetic  control  and  the  other 
mechanical.  The  former  has  been  perfected  to  a  definite  instru- 


1     IliiliillillllillilllllillllliliilillliJllii.ilulllaimliuiii 

Fig.  139.     Leeds  and  Northrup  Recorder. 

ment  and  the  latter  is  still  undergoing  improvement  in  details. 
Both  are  of  the  balance  type  and  give  pen  records  proportional 
to  temperatures  in  rectangular  coordinates;  in  both  the  record- 
ing mechanism  is  entirely  independent  of  the  galvanometer 
system,  and  the  accuracy  uninfluenced  by  changes  in  the  galva- 
nometer constant. 

The  general  appearance  of  the  recorder  with  electromagnetic 
control  is  shown  in  Fig.  139.     As  usually  constructed,  this  re- 


394  HIGH  TEMPERATURES 

corder  is  made  for  any  range  over  100°  F.  to  1800°  F.  with  a 
sensibility  of  ^  per  cent  and  a  constancy  of  repetition  of  f  per 
•cent.  The  pen  will  follow  temperature  changes  at  the  rate  of 
^  per  cent  of  the  total  range  per  second,  giving  a  continuous 
terraced  record. 

The  complete  recorder  consists  of  four  essential  parts:  a  dif- 
ferential galvanometer,  with  a  plunger  magnet  attached  to  it, 
for  operating  periodically  the  contactor;  a  clock  which  drives 
the  paper  and  operates  the  contacting  plunger  magnet;  a  sim- 
ple mechanism  consisting  of  a  screw  and  two  electromagnets  of 
the  plunger  type  for  traveling  the  pen  and  the  balancing  con- 
tact; and  a  paper  drive,  moved  by  the  clock,  which  feeds  a  con- 
tinuous band  of  paper  off  a  roll  and  over  an  apron  carrying  it 
under  an  ordinary  stylographic  pen. 

The  mechanical  recorder  is  of  the  same  general  design,  except 
that  the  moving  mechanisms  are  all  driven  mechanically,  with 
the  added  factor  that,  according  as  the  galvanometer  needle  is 
more  or  less  deflected,  the  pen  mechanism  will  be  driven  faster 
or  slower.  This  permits  following  much  more  rapid  tempera- 
ture changes  than  with  the  electromagnetically  controlled  re- 
corder. 

Both  of  these  instruments  may  also  be  arranged  for  use  with 
thermocouples;  and  the  mechanical  recorder  has  besides  been 
adapted  to  record  directly  the  differential-temperature  curve 
(page  382)  by  means  of  a  double  galvanometer  system,  the  paper 
moving  proportionally  to  the  temperature  of  the  sample  and  the 
pen  proportionally  to  the  difference  in  temperature  between 
the  sample  and  neutral. 

Carpentier's  Electrothermal  Recorder.  —  This  system  gives  rec- 
tilinear coordinates  by  the  use  of  a  cylindrical  frame  contain- 
ing the  paper,  which  advances  continuously  and  against  which 
the  pen  carried  by  the  galvanometer  boom  impinges  periodically 
by  means  of  the  following  electrothermal  mechanism:  The 
galvanometer  boom  carries  a  fine  wire  of  platinum-silver  through 
which  an  electric  current,  either  direct  or  alternating,  may  be 
.sent  from  an  auxiliary,  or  the  ordinary  lighting  circuit.  The 


RECORDING   PYROMETERS  395 

recording  pen  and  wire  are  so  attached  to  the  end  of  the  boom 
by  a  spring  that  when  no  current  is  passing  in  the  wire  the  pen 
is  free  of  the  paper;  but  the  passage  of  a  current  weakens  the 
tension  of  the  wire,  due  to  its  expansion,  and  the  spring  then  pulls 
the  pen  gently  against  the  paper  without  shock.  With  a  suit- 
able commutator,  as  many  as  four  contacts  per  second  may  be 
had  on  the  paper;  and  where  several  records  are  to  be  taken  on 
the  same  sheet  for  as  many  separate  thermometers,  the  same 
principle  of  electrothermal  control  may  be  applied  also  to  the 
commutator,  giving,  when  the  contact  surfaces  are  so  spaced, 
dots  or  dashes  of  different  length  for  each  circuit. 

This  type  of  recorder  and  commutator  may  evidently  be  used 
with  any  kind  of  pyrometer  whose  readings  may  be  taken  with 
a  galvanometer. 

Thermoelectric  Recording  Pyrometer.  —  We  shall  group  the 
thermoelectric  recording  methods  according  to  the  type  of  curve 
which  they  register,  —  temperature-time,  differential,  and  tem- 
perature-rate, —  all  of  which  may  be  realized  both  with  photo- 
graphic and  autographic  registering  apparatus.  Due  perhaps 
to  the  general  convenience  of  the  thermocouple  for  temperature 
measurements,  there  appears  until  very  recently  to  have  been 
more  attention  paid  to  the  development  of  recording  methods 
for  this  type  of  pyrometer  than  for  any  other,  and  this  in  spite 
of  the  fact  that  the  thermocouple  labors  under  the  disadvantage, 
as  compared  with  the  resistance  pyrometer,  for  instance,  in 
having  intrinsically  very  little  energy  available  for  the  mechanical 
operation  of  a  recorder. 

It  would  seem  to  be  mainly  for  this  reason  that  the  early 
thermoelectric  recorders  were  photographic  instruments;  and,  in 
fact,  it  is  only  in  recent  years  that  satisfactory  autographic  ther- 
moelectric recorders  have  been  devised,  and  they  employ  various 
artifices  to  maintain  the  sensitiveness  of  the  registering  galva- 
nometer, necessitating  usually  an  intermittent  or  dotted  record, 
at  least  with  Pt-Rh  thermocouples.  The  difficulty  of  the 
problem,  in  this  case,  is  emphasized  when  it  is  recalled  that  only 
very  weak  currents  can  be  had;  thus  for  a  precision  of  10°  C.  an 


396  HIGH  TEMPERATURES 


apparatus  sensible  to  ^  oTo"o  vo^  *s  necessary,  and  as  the  galva- 
nometer should  have  at  least  100  ohms  resistance,  as  previously 
explained,  when  the  deflection  method  is  used,  the  corresponding 
current  will  be  only  a  millionth  of  an  ampere. 

In  those  cases  where  it  is  permissible,  as  in  many  industrial 
measurements,  to  use  certain  alloys  of  the  base  metals  which 
develop  large  E.M.F.'s,  and  whose  resistance  is  so  low  that  the 
galvanometer  resistance  may  also  be  reduced,  it  is  possible  to 
realize  pen  recorders  giving  a  strictly  continuous  record  curve 
of  a  precision  sufficient  for  many  technical  operations. 

Temperature-rate  Recorders.  —  As  we  have  seen,  it  is  some- 
times of  interest,  more  particularly  in  the  laboratory,  to  measure 
directly  the  speed  of  the  occurrence  of  certain  phenomena,  as 
the  rate  of  cooling  or  chemical  transformation.  We  require  an 
apparatus  that  will  give  the  rate  of  change  of  temperature  of  the 
sample  in  terms  of  its  temperature. 

Le  Chatelier's  Experiments.  —  Le  Chatelier  used  this  method 
in  1887  m  his  study  of  the  properties  of  clays.  He  was  also  the 
first  to  employ  a  photographic  apparatus  for  the  recording  of 
cooling-  or  heating-curve  data,  using  an  arrangement,  to  be  de- 
scribed, in  which  the  photographic  plate  remained  stationary. 

A  luminous  beam  reflected  by  the  galvanometer  mirror  falls 
periodically  at  regular  intervals,  of  a  second  for  instance,  upon 
a  fixed  sensitive  plate.  The  distance  apart  of  two  successive 
images  gives  the  variation  of  temperature  during  unit  time,  that 
is,  the  rate  of  heating  or  of  cooling;  the  distance  from  the  same 
image  to  the  image  corresponding  to  the  beginning  of  the  heating 
will  give  the  measurement  of  the  temperature. 

In  all  cases  of  photographic  recording  it  is  well  to  replace  the 
ordinary  galvanometer  mirrors,  which  give  images  quite  insuffi- 
cient as  to  definition  and  brightness,  by  special  mirrors  made  of 
a  plano-convex  lens,  silvered  on  the  plane  surface.  These  mir- 
rors are  slightly  heavier  than  parallel-face  mirrors,  but  have  two 
important  advantages:  the  absence  of  extra  images  reflected  by 
the  front  surface  of  the  mirror,  and  a  greater  rigidity,  which 
obviates  accidental  bendings  of  the  mirror  arising  from  the  attach- 


RECORDING   PYROMETERS 


397 


ments  to  its  support.  One  may  easily  get  good  mirrors  of  this 
type  of  20  mm.  diameter,  and  with  more  difficulty  of  30  mm. 
diameter.  These  last  give  nine  times  more  light  than  the  mirrors 
ordinarily  employed.  It  is  easy  to  so  choose  the  lens  as  to  give 
a  mirror  of  desired  focal  length.  A  plano-convex  lens  whose 
principal  focus  by  transmission  is  i  m.  will  give,  after  silvering 
the  plane  surface,  an  optical  system  equivalent  to  a  spherical 
mirror  whose  radius  of  curvature  would  be  i  m. 

Le  Chatelier  used  discontinuous  recording.  In  this  manner 
of  recording,  the  luminous  source  should  possess  periodic  varia- 
tions; one  of  the  simplest  to  employ  is  the  electric  spark  between 
two  metallic  points.  The  interruption  of  the  current  is  pro- 
duced by  a  pendulum  at  definite  intervals  of  time. 

In  order  to  have  a  spark  sufficiently  bright,  it  is  necessary  to 
use  an  induction  coil  so  worked  as  to  give  freely  sparks  of  50  mm., 
and  to  reenforce  it  by  a  Leyden  jar  which  reduces  the  length  of 
these  sparks  to  5  mm.;  it  suffices  for  this  to  use  a  jar  of  i  to  2 
liters.  The  choice  of  metals  for  the  points  is  equally  important; 
zinc,  aluminium,  and  especially  magnesium  give  sparks  that  are 
very  photogenic.  These  metals  possess  the  disadvantage  of 
oxidizing  quite  rapidly  in  the  air,  so  that 
it  is  necessary  from  time  to  time  to  clean 
the  points  with  a  file.  The  metallic  sticks 
may  have  5  mm.  diameter,  and  the  dis- 
tance apart  of  the  points  is  2  mm.  One 
might  without  doubt,  using  mercury, 
which  gives  sparks  as  photogenic  as  does 
magnesium,  construct  an  inclosed  ap- 
paratus in  which  the  metal  would  be 
preserved  unchanged. 

To  produce  the  interruption,  there  is 
attached  to  the  pendulum   (Fig.  140)  a 

11.  r     i         i  •  i      j«         •    j  Fig.  140.     Clock 

vertical  platinum  fork  which  dips   into  interrupter 

two  cups  of  mercury  covered  with  alcohol. 

It  is  useful,  in  order  to  reduce  to  a  minimum  the  resistance 
that  the  immersion  of  the  fork  opposes  to  the  motion  of  the 


o 


398  HIGH  TEMPERATURES 

pendulum,  to  place  this  fork  in  the  same  horizontal  plane  as 
the  axis  of  rotation  of  the  pendulum.  In  this  way  one  avoids 
the  translatory  movements  in  the  mercury  which  cause  the  most 
trouble. 

The  only  refinement  with  this  intermittent  lighting  is  to  obtain, 
with  a  spark  much  too  large  and  irregular  to  be  photographed 
directly,  the  illumination  of  a  very  narrow  slit.  It  is  not  sufficient 
to  place  the  spark  behind  the  slit  and  at  a  small  distance  away, 
because  the  slightest  displacement  of  the  spark  would  cause  the 
luminous  beam  to  fall  outside  of  the  mirror  of  the  galvanometer. 
This  difficulty  is  overcome  by  a  well-known  artifice.  A  lens  is 
placed  between  the  electrodes  and  the  mirror  (Fig.  141) ;  the  posi- 
tion of  the  electrodes  is  so  adjusted  that  the  image  of  the  mirror 
is  formed  between  them.  With  a  distance  apart  of  the  electrodes 


Fig.  141.     Focusing  Device. 

of  2  mm.,  a  lens  of  100  mm.  focal  length  and  a  mirror  of  25  mm. 
diameter,  the  image  of  the  latter  will  touch  the  two  points;  the 
spark  then  necessarily  crosses  the  image  of  the  mirror,  and  the 
radiations  passed  by  the  lens  will  fall  certainly  upon  the  mirror. 
One  is  thus  sure  in  placing  before  the  lens  a  fine  metallic  slit  that 
all  the  rays  transmitted  will  reach  the  mirror  and  will  be  sent  to 
the  photographic  plate,  and  that  whatever  may  be  the  position 
of  the  slit  in  front  of  the  lens. 

To  save  time,  it  is  advantageous  to  take  several  sets  of  observa- 
tions on  the  same  plate;  this  is  easily  done  by  arranging  the  plate 
so  that  it  may  be  displaced  vertically  between  two  series,  or  in 
adjusting  the  slit  so  that  it  may  be  moved  similarly  before  the 
lens. 

The  diagram  (Fig.  142)  is  the  reproduction  of  negatives  relative 
to  the  action  of  heat  on  clays.  The  first  line  gives  the  graduation 


RECORDING  PYROMETERS  399 

of  the  couple;  it  has  been  drawn  from  several  different  photo- 
graphs which  have  been  grouped  to  economize  space.  The 
following  lines  are  reproductions  of  negatives  made  photographi- 
cally without  any  intervention  of  the  hand  of  the  engraver.  The 
second  line,  for  example,  represents  the  heating  of  an  ordinary 
clay.  A  slight  contraction  of  the  lines  between  150°  and  350° 
indicates  a  first  phenomenon  with  absorption  of  heat;  it  is  the 
vaporization  of  the  inclosed  water.  A  second  cooling  much 
more  marked  between  550°  and  650°  shows  the  dehydration, 
properly  so  called,  of  the  clay,  the  liberation  of  the  two  molecules 
of  water  in  combination.  Finally,  the  considerable  spacing  of 
the  lines  at  1000°  shows  a  sudden  setting  free  of  heat  correspond- 
ing to  the  isomeric  change  of  state,  after  which  the  alumina  be- 

S  Se 

20°  100°  445°  665° 

I    nil  ffiiiimiiimiiiiiiiiiMiii  iMMiiiiiiiimJ 


HI 

miiiiYiiiiiiiM^ 

Fig.  142.     Heating  Curves  of  Clays. 

comes  insoluble  in  acids.  The  other  rows  refer  to  the  heating  of 
other  varieties  of  clay,  the  third  row  to  kaolin,  the  fifth  to 
steargillite. 

Dejean's  Apparatus.  —  Another  method  of  recording  the  rate 
of  heating  or  cooling  in  terms  of  the  temperature  has  been  de- 
vised by  Dejean.  The  new  feature  of  this  method,  which  gives  a 
continuous  record,  is  the  use  of  an  induction  galvanometer  or 
relay  which  may  be  inserted  in  the  circuit  of  the  more  sensitive 
galvanometer  G\  of  the  Saladin  system  (Fig.  160).  The  principle 
of  the  apparatus  is  shown  in  Fig.  143.  The  induction  relay  is  a 
modified  d'Arsonval  galvanometer  having  an  electromagnet  and 
a  movable  coil,  the  latter  consisting  of  two  distinct  insulated 
windings,  one  of  which  is  connected  to  a  thermocouple.  Heat- 
ing or  cooling  one  junction  of  this  couple  causes  the  coil  to  be 


400  HIGH  TEMPERATURES 

deflected  and  its  motion  in  the  field  of  the  electromagnet  induces 
an  E.M.F.  in  the  second  winding  of  the  coil  which  is  proportional 
to  its  angular  speed  and  hence  to  the  rate  of  change  of  E.M.F. 
of  the  couple,  or  approximately  to  the  rate  of  cooling  or  heating, 

i.e.,  to  — .     The  induced  E.M.F.  is  measured  by  joining  this 
at 

winding  to  the  sensitive  galvanometer  G\.  The  galvanometer 
deflection  passes  through  a  minimum  when  the  heating  or  cooling 
passes  through  a  minimum,  that  is,  for  a  region  in  which  there  is 
an  absorption  or  evolution  of  heat.  A  second  thermocouple  in 
series  with  the  other  galvanometer  Gz  of  the  Saladin  system  gives 
the  temperature  of  the  sample.  We  have,  therefore,  on  the  plate 


Induction 
Galvanometer 


Fig.  143.     Dejean's  Apparatus. 

P  (Fig.  134),  when  the  record  is  taken  photographically,  the  tem- 
peratures as  abscissae  and  the  rate  of  cooling  —  as  ordinates. 

Dejean  has  used  this  method  in  the  study  of  steels  and  has  also 
investigated  with  it  the  copper-cuprous  oxide  system.  The 
transition  temperatures  are  very  sharply  marked.  If  desired, 
direct  reading  may  be  substituted  for  the  photographic  recording, 
with  an  increase  in  precision.  Unless  the  temperature  is  chang- 
ing rapidly,  however,  this  method  lacks  sensitiveness.  It  is 
evidently  a  perfectly  general  method  for  recording  the  rate  of 

change  of  E.M.F.  ~ 


In  neither  Le  Chatelier's  nor  Dejean's  arrangement  can  dif- 
ferences in  the  rate  of  heating  or  cooling  due  to  the  substance 
itself  be  distinguished  from  those  due  to  external  causes,  since  no 
neutral  piece  is  used  (see  page  382). 


RECORDING   PYROMETERS  4OI 

Temperature  Time  Recorders. — There  have  been  a  great  many 
types  of  instruments  constructed  for  recording  temperatures 
directly  in  terms  of  time  for  use  with  thermocouples.  The  early 
forms  were  for  the  most  part  photographic,  giving  continuous 
records,  while  many  of  the  more  recent  ones  are  autographic, 
which  usually  give  discontinuous  results.  We  can  mention  only 
a  few  which  illustrate  sufficiently  well  the  principles  involved. 

The  Apparatus  of  Sir  Roberts- Austen.  —  On  account  of  its  his- 
torical interest  as  well  as  its  intrinsic  usefulness,  we  shall  first 
describe,  with  some  of  its  modifications,  the  photographic  appa- 
ratus of  the  late  Sir  Roberts-Austen,  director  of  the  royal  mint 
at  London. 

A  vertical  slit  lighted  from  a  convenient  source  projects  its 
image,  by  means  of  the  galvanometer  mirror,  on  a  metallic  plate 
pierced  by  a  fine  horizontal  slit,  and  behind  this  slit  moves  a 
sensitive  surface  —  plate  or  paper  —  which  receives  the  luminous 
beam,  defined  by  the  intersection  of  the  horizontal  slit  with  the 
image  of  the  vertical  slit.  If  all  were  at  rest,  the  impression 
produced  by  this  luminous  beam  would  be  reduced  to  a  point. 
If  the  plate  alone  is  moved,  a  vertical  straight  line  will  be  had;  if 
the  galvanometer  mirror  alone  turns,  a  horizontal  line.  Finally, 
the  simultaneous  displacement  of  the  plate  and  mirror  gives  a 
curve  whose  abscissae  represent  temperatures,  and  whose  ordi- 
nates,  time.  The  illumination  of  the  slit  and  the  motion  of  the 
sensitive  surface  may  be  realized  in  many  different  ways. 

Regarding  the  lighting  of  the  slit,  there  are  two  quite  distinct 
cases  to  consider,  —  that  of  laboratory  researches  by  rapid  heating 
or  cooling,  which  last  only  a  few  minutes,  and  that  of  continuous 
recording  of  temperatures  in  industrial  works,  which  may  last 
hours  and  days,  that  is  to  say,  periods  100  times  to  1000  times 
longer.  The  rate  of  displacement  of  the  sensitive  surface,  and 
consequently  the  time  of  exposure  to  the  luminous  action,  may 
vary  in  the  same  ratio.  The  luminous  source  necessary  will  be 
therefore  quite  different,  depending  upon  the  case.  For  very 
slow  displacements  it  is  sufficient  to  use  a  small  kerosene  lamp 
with  a  flame  of  5  to  10  mm.  high.  For  more  rapid  displacements 


402 


HIGH   TEMPERATURES 


use  may  be  made  of  an  ordinary  oil  lamp,  an  Auer  burner,  or  an 
incandescent  lamp;  finally,  for  very  rapid  displacements  of  the 
sensitive  plate,  10  mm.  to  100  mm.  per  minute,  one  may  advan- 
tageously employ  the  oxyhydrogen  flame  or  the  electric  arc.  For 
oxyhydrogen  light  the  most  convenient  is  the  lamp  of  Dr.  Roux, 
with  magnesium  spheres ;  it  consumes  little  gas  and  is  inclosed  in 
a  metallic  box  which  prevents  all  troublesome  diffusions  of  the 
light.  In  more  modern  apparatus  the  Nernst  lamp  is  often  used. 
The  electric  arc  gives  much  more  light  than  is  needed,  and  the 
rapid  wearing  away  of  the  carbon,  by  displacing  the  positions  of 
the  luminous  point,  renders  difficult  the  permanence  of  suitable 
illumination  of  the  slit.  For  very  short  experiments  one  may 
very  conveniently  use  the  mercury  lamp  in  vacuo  (Fig.  144)  or 
the  arc  playing  between  two  mercury  surfaces. 
In  order  to  run  it,  3  amperes  at  30  volts  are 
requisite.  Its  only  disadvantage  is  its  liability  to 
go  out  after  running  a  few  minutes  on  account 
of  the  evaporation  of  the  mercury  in  the  central 
tube.  It  suffices,  it  is  true,  to  give  it  a  slight  jar 
to  make  it  go  again,  by  causing  a  small  quan- 
tity of  mercury  to  pass  from  the  outside  annular 
space  into  the  central  tube.  Special  forms  of 
mercury  lamp  exist,  however,  which  are  free  from 
this  trouble. 

Whatever  the  luminous  source  employed,  the 
slit  may  be  always  lighted  by  means  of  a  lens 
arranged  as  was  indicated  for  discontinuous  recording,  that  is, 
projecting  upon  the  galvanometer  mirror  the  image  of  the 
luminous  source.  When  this  is  large  enough,  it  suffices  to 
place  the  slit  before  the  luminous  source,  bringing  it  up  close 
enough  so  as  to  be  sure  that  some  of  the  luminous  rays, 
passing  through,  fall  upon  the  mirror.  But  there  is  danger 
here  of  so  considerably  heating  the  slit  that  it  may  be  altered: 
for  this  reason  one  is  led  to  use  more  voluminous  light  sources 
than  would  otherwise  be  necessary.  In  the  case  of  the  use  of 
a  lens,  the  useful  luminous  intensity  is  as  great  as  in  placing. 


Fig.  144- 
Mercury  Lamp. 


RECORDING  PYROMETERS  403 

the  slit  immediately  next  to  the  luminous  source,  so  long  as  the 
image  of  the  latter  is  greater  than  the  galvanometer  mirror; 
now  with  the  ordinary  dimensions  of  the  sources  employed  this 
condition  is  always  fulfilled  without  any  special  precaution. 

Instead  of  a  slit  lighted  by  a  distinct  luminous  source,  use  may 
be  made  of  a  platinum  wire,  or  better,  as  does  Charpy,  employ 
a  carbon  filament  of  an  incandescent  lamp  heated  by  an  electric 
current. 

In  order  that  the  line  traced  by  the  recorder  be  very  fine,  it  is 
necessary  that  the  two  slits,  the  luminous  slit  and  the  horizontal 
slit,  be  equally  fine.  Skillful  mechanicians  can  cut  such  slits  in 
metals.  But  it  is  easier  to  make  them  by  taking  a  photographic 
plate  of  bromide  gelatine  that  has  been  exposed  to  the  light,  de- 
veloping until  completely  black,  then  wash  and  dry.  By  cutting 
the  gelatine  with  the  point  of  a  penknife  guided  by  a  ruler,  one 
may  get  transparent  slits  of  a  perfect  fineness  and  sharpness. 

For  sensitive  surfaces,  use  is  made  of  plates  or  films  of  bromide 
gelatine.  Professor  Roberts- Austen  employed  exclusively  plates 
which  permit  more  easily  the  printing  of  a  great  number  of  posi- 
tive proofs.  Charpy,  in  his  researches  on  the  hardening  of  steel, 
made  ,use  of  sensitive  paper,  which  permits  a  much  more  simple 
installation. 

For  industrial  recording,  paper  would  allow  of  the  employing 
large  rolls  lasting  several  days,  as  in  the  recording  magnetic 
apparatus  of  Mascart.  But  in  general  one  wants  to  have  quickly 
the  results  of  the  record;  this  is  always  the  case  in  laboratory 
investigations,  and  almost  always  in  industrial  studies.  It  is 
thus  preferable  to  be  content  with  quite  short  bands  of  paper 
rolled  on  a  cylinder.  "There  exists  such  a  model  quite  well  known 
and  easy  to  use:  the  recording  cylinders  with  an  interior  clock 
movement  of  the  firm  Richard,  Paris.  They  may  be  ordered 
from  the  maker  with  any  desired  rate  of  rotation;  unfortunately, 
this  rate  cannot  be  changed  at  the  pleasure  of  the  operator,  a 
desideratum  in  laboratory  investigations. 

In  the  apparatus  used  by  Charpy,  or  in  its  very  elaborate  form 
as  constructed  by  Toepfer  of  Potsdam,  for  Kurnakow,  the  ver-. 


404 


HIGH  TEMPERATURES 


tically  moving  plate  is  replaced  by  a  rotating  cylinder  wound 
with  the  sensitized  paper  on  which  the  deflections  of  the  galva- 
nometer are  registered.  This  form  of  recorder  had  also  been 
used  and  discarded  by  Roberts-Austen.  Fig.  145  represents  the 
installation  of  the  recording  pyrometer  used  by  Charpy  in  his 
researches  on  the  quenching  of  steel.  To  the  right  is  the  galva- 
nometer, to  the  left  the  Richard  recording  cylinder,  and  in  the 
middle  the  electric  furnace  used  for  heating  the  samples  of  steel. 
It  is  interesting  to  note  in  passing  that  Charpy  was  the  first  to 
use  electric  heating  in  this  kind  of  work.  Kurnakow's  apparatus, 
which  must  be  placed  in  a  dark  room,  is  furnished  with  an 


Fig.  145.     Charpy 's  Apparatus. 

auxiliary  telescope  and  scale  system  using  red  light,  so  that  the 
experiment  may  be  controlled  during  the  taking  of  a  record. 
As  constructed,  five  speeds  may  be  given  to  the  cylinder;  and 
there  is  provided  an  E.M.F.  compensating  system  for  main- 
taining the  maximum  sensibility  over  a  Series  of  temperature 
ranges. 

There  is  another  device,  used  by  C.  L.  A.  Schmidt,  by  which 
the  experiment  may  be  watched  while  a  photographic  record  of  a 
cooling  curve  is  being  taken.  It  consists  in  shunting  the  sensi- 
tive photo-recording  galvanometer  G  (Fig.  146),  in  series  with  a 
high  resistance  R,  across  a  direct-reading  milli voltmeter  V.  If 
the  resistance  of  R  +  G  is  great  compared  with  that  of  V,  the 


RECORDING   PYROMETERS 


405 


readings  of  the  millivoltmeter  will  not  be  altered  appreciably  by 
this  operation.  Schmidt  moves  the  photographic  plate,  mounted 
as  in  the  apparatus  of  Roberts-Austen,  by  means  of  a  screw 
driven  by  a  small  motor.  In  this  way  any  desired  speed  may  be 
given  to  the  plate. 

R 


Fig.  146.     Schmidt's  Device. 

If  plates  are  used,  they  may  be  placed  in  a  movable  frame 
regulated  by  a  clock  movement;  this  is  the  first  arrangement 
employed  by  Professor  Roberts- Austen  (Fig.  147).  But  this  in- 
stallation, somewhat  costly  and  complicated,  has  the  same  dis- 
advantage as  the  recording  cylinders,  in  that  but  a  single  speed 
can  be  given  to  the  sensitive  surface.  In  order  to  drive  the  plate > 


Fig.  147.     Apparatus  of  Roberts-Austen. 

Roberts-Austen  later  used  a  buoyed  system  in  which  the  rate  of 
rise  of  level  of  the  water  is  controlled  at  will  by  the  agency  of  a 
Mariotte's  flask  and  a  simple  water  cock.  The  plate  is  kept  in 
an  invariable  vertical  plane  by  means  of  two  lateral  cleats  whose 
friction  is  negligible  on-  account  of  the  mobility  of  the  float. 
The  sketch  (Fig.  148)  gives  the  arrangement  of  a  similar  appa- 


4o6 


HIGH  TEMPERATURES 


ratus  made  by  Pellin  for  the  laboratory  of  the  College  de  France. 
It  carries  a  13  by  18  cm.  plate  which  is  attached  to  the  float  by 
means  of  two  lateral  springs  not  shown 
in  the  sketch.  Neither  are  the  two 
guides  of  the  float,  immersed  in  water, 
indicated ;  the  play  next  the  cleats  is  only 
two-tenths  of  a  millimeter.  The  uncer- 
tainty that  this  play  can  cause  in  the 
position  of  the  plate  is  quite  negligible. 
The  curve  (Fig.  149)  is  the  reproduction 
of  an  experiment  made  with  such  an 
arrangement  by  Roberts-Austen  on  the 
solidification  of  gold. 

During  the  whole  period  of  freezing  the 
temperature  remained  stationary,  then  lowering  of  temperature 
was  produced  at  a  regularly  decreasing  rate  as  the  temperature 
of  the  metal  approached  that  of  the  surroundings. 


1065°  C. 


—I  —  ' 

'  —  r— 

3- 

ft 



- 

-1 

i 

U                             I/ 

Fig.  148.    Plate-holder. 

12°C. 


Fig.  149.     Record  with  Apparatus  of  Roberts-Austen. 

It  is  indispensable  to  trace,  on  each  sensitive  surface  on  which 
is  to  be  recorded  a  curve,  the  line  corresponding  to  the  surround- 
ing temperature,  or  at  least  a  parallel  reference  line.  This  is 
very  easy  in  the  case  of  the  guided  plate  or  of  the  paper  rolled 
on  a  cylinder.  It  suffices,  after  having'  brought  the  couple  to 
the  temperature  of  its  surroundings,  to  displace  in  the  opposite 


RECORDING   PYROMETERS  407 

direction  the  sensitive  surface;  the  second  curve  traced  during 
this  inverse  movement  is  precisely  the  line  of  the  zero  of  the 
graduation  of  the  temperatures.  But  this  is  a  dependence  that 
may  be  evaded  by  registering  at  the  same  time  as  the  curve  a 
reference  line  by  means  of  a  fixed  mirror  attached  to  the  galva- 
nometer in  the  path  of  the  luminous  beam  which  lights  the  mov- 
able mirror.  Roberts-Austen  likewise  made  use  of  the  luminous 
beam  reflected  by  the  fixed  mirror  to  inscribe  the  time  in  a  precise 
manner.  A  movable  screen  driven  by  a  second  pendulum  cuts 
off  at  equal  intervals  of  time  this  second  luminous  beam.  The 
reference  line,  instead  of  being  continuous,  is  made  up  of  a  series 
of  discontinuous  marks  whose  successively  corresponding  parts 
are  at  intervals  of  one  second,  as  is  shown  in  Fig.  149. 

The  curves  once  obtained  must  be  very  carefully  examined  to 
recognize  the  points  where  the  gradient  presents  slight  anomalies, 
characteristic  of  the  transformations  of  the  body  studied.  Gen- 
erally these  irregularities  are  very  insignificant,  and  it  would  be 
well,  in  order  to  recognize  them  with  certainty,  to  obtain  curves 
traced  on  a  much  greater  scale.  Practically  this  magnification 
is  not  possible  without  auxiliary  devices  which  limit  either  the 
range  or  the  sensibility;  thus  the  sensitiveness  of  the  galvanom- 
eter may  be  increased,  and  so  the  deflection,  but  then  for  the 
greater  range  of  temperature  the  luminous  image  would  fall  off 
the  sensitive  plate. 

In  practice  it  has  been  found  difficult  to  realize  conveniently 
a  sufficiently  steady  motion  of  the  plate  in  the  Roberts-Austen 
system  of  recording,  and  attempts  have  been  made  to  devise 
methods  in  which  the  photographic  plate  remains  fixed  in  posi- 
tion. This  has  been  successfully  accomplished  by  Saladin,  whose 
apparatus  (Fig.  160,  page  419)  has  been  modified  by  Wologdine 
to  give  the  temperature-time  curve  by  removing  the  prism  M 
and  substituting  for  the  second  galvanometer  GI  a  plane  mirror 
turning  about  an  horizontal  axis.  This  mirror  may  be  controlled 
by  an  hydraulic  system  as  in  Roberts-Austen's  apparatus,  or  by 
clockwork  as  in  the  model  constructed  by  Pellin  of  Paris.  The 
deflection  of  the  galvanometer  G\  gives  to  the  beam  of  light  an 


408  HIGH  TEMPERATURES 

horizontal  motion  over  the  plate  proportional  to  the  tempera turer 
while  the  vertical  motion  of  the  beam  of  light  is  given  by  the 
mirror  turning  at  a  uniform  rate,  and  is  therefore  approximately 
proportional  to  the  time  as  registered  on  a  flat  plate. 

Autographic  Recorders.  —  To  obtain  a  satisfactory  autographic 
or  pen  record  with  platinum  thermocouples  without  sacrifice  of 
sensibility  of  the  galvanometer,  it  is  necessary  to  eliminate  the 
friction  of  the  pen  or  stylus  upon  the  paper.  This  has  been 
accomplished  by  the  use  of  mechanisms  which  cause  the  pen  or 
stylus  at  the  end  of  the  galvanometer  boom  to  make  only  momen- 
tary contact  with  the  moving  paper.* 


Fig.  150.     Siemens  and  Halske  Recorder. 

In  the  Siemens  and  Halske  form  of  instrument  (Figs.  150  and 
151),  the  paper  P  is  driven  forward  by  the  same  clockwork  that 
controls  the  pressing  down,  by  means  of  the  arm  B,  of  the  stylus 
N,  which  imprints  dots  periodically  on  the  paper  by  means  of  a 
typewriter  ribbon  running  across  and  beneath  the  record  sheet. 
This  system  permits  of  taking  a  record  continuously  over  very 

*  There  are  a  considerable  number  of  thermoelectric  recorders.  Among 
the  manufacturers  of  these  instruments  are:  Siemens  and  Halske,  Berlin; 
Hartmannand  Braun,  Frankfort  a.  M.;  Pellin,  Chauvin  and  Arnoux,  Carpen- 
tier,  and  Richard,  Paris;  Leeds  and  Northrup,  the  Thwing  Instrument  Com- 
pany, and  Queen  of  Philadelphia;  the  Scientific  Instrument  Company  of 
Cambridge,  England,  and  Rochester,  N.  Y.;  the  Bristol  Company,  Water- 
bury,  Conn. 


RECORDING  PYROMETERS 


409 


long  periods  of  time.  In  mos<:  of  the  other  recorders  the  paper 
is  wound  upon  a  drum,  and  various  devices  are  used  to  obtain 
the  record ;  thus  in  the  Hartmann  and  Braun  type  a  silver  stylus 
makes  sulphide  dots  on  a  prepared  paper,  and  in  the  Cambridge 
thread  recorder  rectangular  coordinates  are  obtained  by  having 
the  galvanometer  boom  strike  an  inked  thread  which  rui 
parallel  to  the  drum  (Fig.  152). 


Fig.  151.     Principle  of  Recorder. 


A  Siemens  and  Halske  drum  recorder  with  pivot  galvanometer, 
suitable  for  technical  work,  and  which  is  inclosed  in  a  dustproof 
metallic  case,  is  shown  in  Fig.  153.  It  may  be  adjusted  for 
seven-day  records. 

As  previously  stated,  these  autographic  instruments  all  give 
intermittent  records  for  the  platinum  thermocouples,  and  are 
limited  to  one  or  two  speeds;  and  although  they  may  be  made 
very  sensitive  they  are  not  adapted  for  the  detection  of  trans- 
formations which  take  place  very  rapidly,  since  the  recording 


4io 


HIGH  TEMPERATURES 


Fig.  152.     Thread  Recorder. 


Fig.  153.     Drum  Recorder. 


RECORDING   PYROMETERS 


411 


interval  cannot  readily  be  shortened  much  below  10  seconds, 
and  in  most  instruments  this  interval  is  greater  than  15  seconds. 
In  other  words,  they  can  be  used  advantageously  only  for  slow 
cooling  or  heating. 

A  continuous  pen  record  may  be  obtained  with  galvanometers 
suited  for  use  with  the  base-metal  couples  developing  high 
E.M.F.'s,  such  as  the  Bristol,  Hoskins,  Thwrng,  etc. 

In  order  to  eliminate  the  effect  of  irregularity  of  outside  con- 
ditions which  influence  the  rate  of  cooling,  a  method  commonly 


Time 


Fig.  154.     Furnace  and  Charge  Temperature  Curves. 

used  when  endeavoring  to  detect  small  transformations  consists 
in  placing  a  second  thermocouple  in  the  furnace,  but  sufficiently 
removed  from  the  substance  studied  to  be  uninfluenced  by  its 
behavior.  Alternate  readings  on  the  temperature  of  the  test 
piece  (0)  and  of  the  furnace  (0')  are  then  taken,  preferably  at 
definite  time  intervals.  The  data  are  most  readily  discussed  by 
plotting  the  two  temperature-time  curves  side  by  side  as  shown 
in  Fig.  154,  or  by  plotting  the  difference  in  temperature  0  —  6' 
against  the  temperature  0  of  the  test  piece. 
This  method  may  be  made  recording  either  by  using  two  in- 


412 


HIGH  TEMPERATURES 


struments  or  by  modifying  one  of  the  above-mentioned  auto- 
graphic recorders  so  as  to  trace  the  curves  of  two  thermocouples 
on  the  same  sheet.  In  practice,  however,  this  method  is  usually 
resorted  to  only  when  great  sensibility  is  desired,  as  in  detecting 
minute  internal-energy  changes,  when  the  potentiometer  com- 
bined with  the  deflection  galvanometer  is  the  most  sensitive  and 
quick- working  arrangement  for  taking  the  measurements.  It  is 
convenient  to  use  thermocouples  of  the  same  composition  so  as 


Fig.  155.     Brearley  Curve  Tracer  and  Accessories. 


to  have  readings  of  both  the  temperature  of  the  sample  and  of 
the  furnace  given  by  the  same  potentiometer  setting,  and  so 
depend  upon  the  galvanometer  deflections  for  measuring  the 
residual  parts  of  6  and  d' '.  ' 

Regarding  the  precision  of  this  method,  it  is  to  be  noted  that 
the  quantity  it  is  really  desired  to  measure  is  6  —  6'  in  terms  of  6, 
and  this  is  accomplished  by  measuring  6  and  0',  hence  the  sensi- 
bility of  6  —  6'  is  no  greater  than  that  of  6  or  0'.  In  other  words, 
the  method  requires  the  maximum  refinement  of  measurement 


RECORDING   PYROMETERS 


413 


to  obtain  the  quantity  sought,  as  well  as  the  maximum  of  com- 
putation or  plotting  to  reduce  the  observations. 

Semi-automatic  Recording.  —  The  Brearley  curve  tracer,  man- 
ufactured by  the  Cambridge  Company,  is  a  semi-automatic  ap- 
paratus for  registering  the  time-temperature  curve.  As  shown 
with  accessories  in  Fig.  155,  a  small  tube  furnace  is  connected  to 
an  electric  supply  main  so  as  to  heat  the  specimen  within  the  fur- 
nace; a  platinum-iridium  couple,  whose  hot  junction  is  within  the 
specimen,  is  connected  in  series  with  a  resistance  and  moving  coil 
galvanometer  of  adjustable  sensibility;  a  Nernst  lamp  furnishes 


-5" 
-4" 
-3" 
-2" 


Fig.  156.     Curves  with  Brearley  Apparatus. 

illumination  and  gives  a  sharp  image  at  M  on  the  scale  G.  The  ro- 
tating drum  L  is  surmounted  by  a  sliding  carriage  N  carrying  two 
pointers,  one  of  which,  M,  is  fixed,  and  the  second,  immediately 
below,  carries  a  pen  and  is  depressed  every  second  on  the  paper 
wound  on  the  drum.  The  pointer  M  is  made  to  fojlow  the  spot 
of  light  by  the  operator  by  turning  a  handle  at  the  end  of  a  long 
screw  on  which  N  runs.  There  is  electromagnetic  clock  control 
of  the  drum  and  of  the  pen.  The  record  is.  therefore  a  series  of 
dots  one  second  apart.  The  temperature  scale  may  be  made  as 
open  as  desired,  and  a  complete  heating  and  cooling  curve  for  a 
steel  sample  may  be  obtained  in  a  few  minutes.  This  instru- 


414  HIGH  TEMPERATURES 

ment  is  also  now  made  to  give  a  continuous  record.  A  sample 
curve  is  shown  in  Fig.  156. 

Another  method  of  working,  in  which  the  apparatus  is  com- 
pletely autographic  for  relatively  short  temperature  intervals, 
and  at  the  same  time  very  sensitive,  is  to  use  a  recording  galva- 
nometer in  connection  with  a  potentiometer.  This  requires  the 
operator  to  adjust  the  dials  of  the  potentiometer  to  step  from 
one  temperature  interval  to  the  next,  these  intervals  varying  in 
length  with  the  galvanometer  sensibility,  which  should  be  capable 
of  adjustment  to  give  longer  or  shorter  temperature  intervals. 

Differential  Curves.  —  The  method  of  page  411  may  readily  be 
modified  so  as  to  give  6  —  0',  the  difference  in  temperature  be- 
tween the  test  piece  and  furnace,  by  direct  measurement  instead 
of  by  computation,  with  the  added  advantage  that  the  precision 


Fig.  157.    Method  of  Burgess. 

of  B  —  0'  may  be  made  very  great  as  compared  with  that  of  0, 
the  temperature  of  the  sample.  This  may  be  accomplished,  for 
example,  by  placing  a  commutator,  which  may  be  driven  by  a 
clock  mechanism,  in  the  thermocouple  circuit  at  A,  Fig.  157,  so 
that  alternate  measurements  on  0  and  0  —  0'  may  be  taken  in 
terms  of  the  time.  Evidently  the  connections  may  be  made  so 
that  either  the  galvanometer  G2  of  the  same  direct-reading  or 
potentiometer  system  that  measures  0,  or  a  separate  instrument 
GI,  as  shown  in  the  figure,  may  be  used  to  measure  0  —  0' '.  Both 
galvanometers  may  be  photographic  or  autographic  recorders. 

Use  of  a  Neutral  Body.  —  Accidental  variations  in  the  indica- 
tions of  the  auxiliary  thermocouple  giving  0',  the  furnace  tem- 
perature, may  largely  be  eliminated  by  placing  this  couple  within 
a  blank  or  neutral  substance.  The  material  of  the  neutral  body 
should  be  such  that  it  undergoes  no  transformations  involving 


RECORDING   PYROMETERS 


415 


an  absorption  or  evolution  of  heat  within  the  temperature  range 
studied,  such  as  a  piece  of  platinum,  porcelain,  or  even  in  some 
cases  nickel  or  nickel  steel.  It  is  also  desirable  that  the  sample 
and  neutral  have  as  near  as  may  be  the  same  heat  capacities  and 
emissivities.  The  sample  to  be  studied  and  the  neutral  piece  are 
placed  near  together  and  arranged  symmetrically  with  respect  to 
the  temperature  distribution  within  the  furnace. 

To  Roberts-Austen  again  was  due  the  credit  of  first  devising 
a  sensitive  differential  method  using  the  neutral  body.  He  also 
modified  his  photographic  recorder  (Fig.  147)  so  as  to  give,  by 
means  of  a  second  galvanometer,  the  6  —  0'  vs  t  curve  on  the  same 
plate  with  the  0  vs  t  curve,  from  which  a  curve  giving  6  —  Bf  in 


Fig.  158.     Use  of  Neutral,  Roberts- Austen. 

terms  of  0  could  be  constructed.  His  arrangement  of  the  direct- 
reading  and  differential  thermocouple  and  galvanometer  circuits 
is  shown  in  Fig.  158  in  which  S  is  the  sample  or  test  piece,  and  N 
the  neutral  body  possessing  no  transformations;  the  galvanometer 
G2  measures  the  temperature  6  of  the  sample/and  GI  measures  the 
difference  in  temperature  0  —  6'  between  the  sample  and  the 
neutral.  Curves  for  steels  and  alloys  were  usually  taken  with 
the  samples  in  vacuo. 

It  is  evident  that  Roberts-Austen's  final  photographic  appa- 
ratus, although  very  sensitive,  was  also  complicated  and  very 
delicate  of  adjustment,  and  in  practice  it  took  great  skill  in  its 
use,  requiring  for  instance  some  three  or  four  successive  exposures 
adjusted  to  the  proper  adjacent  temperature  ranges,  to  take  the 
cooling  curve  of  a  steel  from  1100°  to  200°  C. 


4i6 


HIGH  TEMPERATURES 


Most  of  the  recent  exact  work  employing  the  principle  of  this 
method  has  been  done  by  taking  the  observations  of  6  directly 
on  a  potentiometer  and  6  —  0'  on  the  same  or  an  auxiliary  galva- 
nometer. In  this  case  of  direct  reading,  the  simpler  arrangement 
of  thermocouples  indicated  in  Fig.  157,  due  to  Burgess,  may  ad- 
vantageously replace  Roberts- Austen's  (Fig.  158),  or  the  modifica- 
tion shown  in  Fig.  159,  such  as  used  by  Carpenter  and  others. 
The  first  dispenses  with  one  thermocouple  and  the  drilling  of  a 
second  hole  in  the  sample. 

This  method  is  evidently  capable  of  attaining  maximum  sen- 
sitiveness, since  the  galvanometer  connected  to  the  differential 
thermocouple,  giving  9  —  0'  vs  t,  may  be  made  as  sensitive  as 


c  c 

Fig.  159.     Arrangement  used  by  Carpenter. 

desired  independently  of  the  6  vs  t  system.  There  is  the  further 
advantage  that  no  limits  are  set  to  the  range  of  temperatures  over 
which  a  given  precision  in  0  —  d'  may  be  had.  There  is,  however, 
a  limitation  on  the  certainty  of  interpretation  of  results  by  this 
method,  especially  when  the  rate  of  cooling  is  rapid,  due  to  the 
fact  that  it  is  practically  impossible  to  realize  the  ideal  condition 
of  having  6  —  tf  =  a  constant,  or  keeping  the  cooling  curves  of 
the  test  piece  and  neutral  parallel  for  temperature  intervals 
within  which  there  are  no  transformations  of  the  test  piece.  The 
rate  of  cooling,  and  hence  the  value  of  8  —  d',  is  influenced  by 
several  factors,  among  the  most  important  of  which  are  the  mass 
of  each  substance,  —  the  unknown  and  the  neutral,  —  its  specific 
heat,  conductivity,  and  emissivity,  as  well  as  the  relative  heat 
capacities  of  the  furnace  and  inclosed  samples.  The  0  —  tf  vs  t 


RECORDING   PYROMETERS  417 

line  is,  however,  always  a  smooth  curve,  except  for  the  regions 
in  which  there  are  transformations  in  the  substance  under  study. 

The  autographic  system  of  recording  may  also  be  used,  and  it  is 
possible  to  construct  an  apparatus  by  means  of  which  both  the 
0  vs  t  and  6  —  d'  vs  t  curves  shall  be  recorded  simultaneously  on  the 
same  sheet  by  the  same  galvanometer  boom.  In  order  to  accom- 
plish this,  we  have  made  use  of  a  Siemens  and  Halske  recording 
millivoltmeter  having  a  total  range  of  1.5  millivolts  and  a  resist- 
ance of  10.6  ohms.  The  E.M.F.  generated  by  the  differential 
thermocouple,  proportional  to  6  —  0',  is  recorded  directly  by  this 
instrument.  i°  C.  corresponds  to  from  16  to  19  microvolts 
between  300°  and  1100°  C.  for  a  platinum-iridium  couple,  or  to 
about  1.8  mm.  on  the  record  paper.  In  series  with  the  Pt-Ir 
thermocouple  giving  temperatures  is  a  suitable  resistance,  about 
200  ohms  in  this  case,  so  that  the  galvanometer  boom  may  be 
kept  within  the  limits  of  the  paper  when  recording  values  of  9. 
The  circuit  is  made  alternately  through  the  direct  and  the  differ- 
ential thermocouple  circuits  in  series  with  the  recorder  by  means 
of  a  polarized  relay  actuated  by  the  same  battery  that  depresses 
the  galvanometer  boom  when  the  mark  is  made  on  the  paper. 
The  thermocouple  circuits  may  be  those  of  either  Figs.  157,  158, 
or  159,  but  with  the  galvanometer  G2  indicating  temperatures 
suppressed. 

It  is  evident  that  by  recording  the  two  curves,  0  —  6f  vs  t  and 
0  vs  t,  on  the  same  sheet  there  is  some  sacrifice  in  the  ability  to  de- 
tect small  and  rapid  transformations,  since  the  spacing  is  doubled. 
Usually  also,  with  such  an  arrangement,  the  galvanometer  will 
not  be  completely  aperiodic  for  one  or  the  other  system.  On 
the  other  hand,  it  is  of  great  advantage  to  have  the  curves  to- 
gether and  obtained  independently  of  inequalities  in  clock  rates, 
which  are  a  serious  source  of  error  in  locating  transformation 
points  exactly  when  two  separate  instruments  are  used.  The 
same  result  may  be  effected  by  shunting  the  galvanometer  when 
on  the  temperature  side.  This  of  course  cuts  down  very  greatly 
the  resistance  of  the  thermocouple  circuit,  a  disadvantage  unless 
a  sensitive  galvanometer  of  high  resistance  is  used.  Such  gal- 


418  HIGH  TEMPERATURES 

variometers  suitable  for  mechanical  recording  are  not  yet  avail- 
able. In  Thwing's  recording  pyrometer,  two  galvanometers, 
one  giving  temperatures  and  the  other  differences,  impress  their 
records  on  a  single  chart  driven  by  one  clock. 

When  it  is  desired  merely  to  detect  the  existence  of  a  transfor- 
mation without  measuring  its  temperature  exactly,  the  sensitive 
form  of  recording  millivoltmeter  may  be  connected  directly  to 
the  differential  thermocouple  without  other  accessories,  as  was 
done  by  Hoffmann  and  Rothe  in  studying  the  transformations 
of  liquid  sulphur. 

Salaam's  Apparatus.  —  It  is  sometimes  of  advantage  to  be 
able  to  record  and  discuss  the  data  independently  of  the  time, 
and  so  express  6  —  8',  the  difference  in  temperature  between 
sample  and  neutral,  directly  in  terms  of  0,  the  temperature  of  the 
sample.  This  may  evidently  be  accomplished  by  replotting 
the  results  obtained  from  the  curves  of  the  previous  differential 
methods  which  involve  the  time.  It  was  reserved,  however,  to 
Saladin,  engineer  of  the  Creusot  Works,  to  invent,  in  1903,  a 
method  that  would  record  photographically  the  6  vs  B  —  0'  curve 
directly,  thus  obviating  any  replotting.  His  method  possesses 
also  the  advantage  of  having  the  photographic  plate  fixed  in 
place.  The  forms  of  curve  obtained  in  this  way  are  illustrated 
in  Fig.  134. 

The  arrangement  of  the  apparatus  in  its  simplest  form,  due  to 
Le  Chatelier,  is  shown  in  Fig.  1 60.  Light  from  the  source  S  strikes 
the  mirror  of  the  sensitive  galvanometer  G\  whose  deflections 
measure  the  differences  in  temperature  (6  —  B'}  between  the 
sample  under  study  and  the  neutral  body.  The  horizontal 
deflections  of  the  beam  of  light  are  now  turned  into  a  vertical 
plane  by  passing  through  the  totally  reflecting  prism  M  placed  at 
an  angle  of  45  degrees.  A  second  galvanometer  G2,  whose  deflec- 
tions are  a  measure  of  the  temperature  of  the  sample  and  whose 
mirror  in  its  zero  position  is  at  right  angles  to  that  of  Gi,  reflects 
the  beam  horizontally  upon  the  plate  at  P.  The  spot  of  light  has 
thus  impressed  upon  it  two  motions  at  right  angles  to  each  other, 
giving,  therefore,  on  the  plate  a  curve  whose  abscissae  are  ap- 


RECORDING   PYROMETERS 


419 


proximately  proportional  to  the  temperature  6  of  the  sample  and 
whose  ordinates  are  proportional  to  6  —  0'.  The  sensitiveness  of 
the  method  depends  upon  that  of  the  galvanometer  Gi,  which 
may  readily  be  made  to  give  5  or  6  mm.  for  each  degree  cen- 


Pt.— Rh. 
Fig.  1 60.     Saladin's  Apparatus. 

tigrade.  The  arrangement  of  the  thermocouple  circuits  is  the 
same  as  in  Figs.  158  or  159.  If  so  desired,  the  time  may  also  be 
recorded  by  means  of  a  toothed  wheel  driven  by  a  clock  and 
placed  in  the  path  of  the  beam  of  light.  Compact  forms  of  this 
apparatus,  which  is  used  considerably  in  metallurgical  labora- 


420  HIGH   TEMPERATURES 

tories,  are  made  by  Pellin,  Paris,  and  by  Siemens  and  Halske, 
Berlin.  The  lens  between  G\  and  G2  may  be  suppressed. 

When  steels  and  metallic  alloys  in  the  solid  state  are  being 
investigated,  advantage  may  be  taken  of  the  thermoelectric 
behavior  of  the  sample  itself  to  record  the  critical  regions  with 
Saladin's  apparatus.  Thus  Boudouard  measures  0  —  0'  by  means 
of  platinum  wires  set  into  crevices  at  each  end  of  the  sample, 
taking  advantage  of  the  fact  that  the  transformation  will  usually 
be  progressive  along  the  sample.  This  modification  eliminates 
the  neutral  piece  and  one  platinum  or  alloy  wire,  but,  as  Le 
Chatelier  has  shown,  is  less  accurate  than  the  usual  form  of 
Saladin's  apparatus;  and  its  indications  may  even  be  indeter- 
minate or  ambiguous,  as  the  reaction  may  start  midway  between 
the  embedded  wires  or  at  either  end. 

Saladin's  method,  it  should  be  noted,  is  a  perfectly  general  one 
for  recording  the  relations  between  any  two  phenomena  which 
may  be  measured  in  terms  of  E.M.F.  or  as  the  deflections  of 
two  galvanometers.  The  Leeds  and  Northrup  Company  have 
recently  modified  their  autographic  recorder,  p.  393,  to  trace 
the  6  vs  6  —  &'  curve,  using  several  differential  couples  in  series 
in  order  to  obtain  the  required  sensibility. 

Registration  of  Rapid  Cooling.  —  None  of  the  experimental 
arrangements  so  far  described  is  adapted  for  measuring  the  very 
rapid  cooling,  i.e.,  several  hundred  degrees  in  a  few  seconds,  met 
with  in  such  processes  as  quenching  or  chilling.  The  develop- 
ment of  methods  for  measuring  rapidly  varying  temperatures 
will  undoubtedly  be  of  great  use  in  the  solution  of  many  physical 
and  metallurgical  problems  involving  products  whose  properties 
depend  on  cooling  velocities.  Only  a  few  preliminary  investi- 
gations into  this  field,  however,  have  as  yet  been  made. 

Le  Chatelier's  Experiments.  —  Le  Chatelier,  in  an  investigation 
of  the  quenching  of  small  samples  of  steel,  and  the  effect  of  various 
baths,  made  use  of  a  galvanometer  having  a  period  of  0.2  second 
and  a  resistance  of  7  ohms,  whose  deflections,  produced  by  the 
current  from  a  thermocouple  inserted  into  the  specimen  under- 
going the  quenching,  were  recorded  on  a  photographic  plate 


RECORDING   PYROMETERS  421 

moving  vertically  at  a  speed  of  3  mm.  per  second.  A  half- 
second's  pendulum  vibrating  across  the  path  of  the  beam  of 
light,  from  a  Nernst  glower  as  source,  gave  a  measure  of  the  time. 
He  succeeded  in  recording  satisfactorily  temperature  intervals 
of  700°  C.  in  6  seconds,  using  as  samples  cylinders  18  mm.  on 
a  side,  and  obtained  results  of  great  interest  to  the  theory  and 
practice  of  hardening  steel  samples  by  quenching  in  baths  of 
various  kinds  of  liquids.  Le  Cha teller  recognized  the  desira- 
bility of  increasing  the  precision  and  sensitiveness,  and  of 
improving  the  technique,  of  this  method,  and  suggested  the 
advantages  of  using  for  the  registration  an  oscillograph  arrange- 
ment, or  a  string  galvanometer  of  very  short  period  such  as 
Enithoven's,  in  which  the  displacements  of  a  silvered  quartz 
fiber  of  high  resistance  in  an  intense  magnetic  field  are  measured 
photographically. 

Benedicks'  Experiments.  —  Following  the  suggestions  of  Le 
Chatelier,  Benedicks  has  carried  out  a  series  of  researches  on  the 
cooling  power  of  liquids,  on  quenching  velocities,  and  on  certain 
constituents  of  steel.  The  errors  in  cooling  curves  of  metals 
have  also  been  studied  recently  by  Hayes. 

Benedicks'  apparatus,  as  arranged  for  taking  the  time-tem- 
perature curve  of  steel  samples  during  quenching,  is  shown  in 
Fig.  161.  The  principles  here  applied  may  evidently  be  used 
in  other  kinds  of  experimentation  involving  rapid  cooling. 

The  specimen  A  is  heated  in  a  small  electric  furnace  B,  which 
is  provided,  in  its  lower  part,  with  a  narrow  opening  parallel  to 
the  longitudinal  axis,  through  which  passes  a  holder  C,  which 
turns  about  an  horizontal  axis,  being  given  a  definite  torque  by 
a  spiral  spring  D,  and  maintained  vertical  by  an  electromagnetic 
control  E.  Through  a  bore  in  C  a  thermocouple  is  led  into 
the  interior  of  A,  and  the  cold  junction  is  contained  in  the 
ice  box  F,  from  which  the  wires  are  led  to  a  commutator  G 
by  means  of  which  either  the  thermocouple  A  or  the  calibrat- 
ing apparatus  b,  c,  etc.,  may  be  connected  to  the  measuring 
instrument  /,  this  being  a  small  string  galvanometer  by  Edel- 
mann  of  Munich.  The  light  from  the  arc  lamp  K,  passing 


422 


HIGH  TEMPERATURES 


through  the  microscope  of  the  galvanometer,  fitted  with  a 
projection  eyepiece,  gives  an  image  of  the  movable  string  on 
the  registration  apparatus  L,  provided  with  a  rotating  cylinder 
which  carries  the  sensitive  paper. 


Fig.  161.     Apparatus  of  Benedicks. 

Finally,  the  electromagnet  release  E  is  connected  with  an 
accumulator  N  and  a  contact  T  on  the  shutter  before  the  cylin- 
der L. 

The  process  of  registration  is  therefore  as  follows :  The  cylinder 
L  is  set  in  rotation,  and  as  the  edge  of  the  sensitive  paper  passes 


RECORDING   PYROMETERS  423 

the  window  T  the  shutter  rises.  At  the  same  instant  the  cir- 
cuit of  E  is  closed,  releasing  the  arm  C.  This  automatically 
and  quickly  quenches  the  specimen  A  in  the  cistern  of-  water  M 
beneath. 

Precautions  have  to  be  taken  in  insulating  the  thermocouple 
wires  leading  into  the  specimen  and  in  insuring  good  contact 
of  the  couple  junction  against  the  specimen  with  a  water-tight 
joint.  Capillary  tubes  of  fused  quartz,  which  will  also  stand 
sudden  temperature  changes,  were  used  for  insulating,  and  water 
was  prevented  from  entering  joints  by  means  of  compressed  air 
introduced  into  the  containing  tube  C  of  the  thermocouple  wires. 

The  calibrating  apparatus  consists  of  a  sliding  commutator 
c,  b,  the  blocks  of  which  are  connected  to  fixed  points  on  a  slide 
wire  r  in  such  positions  as  to  give  electromotive  forces  corre- 
sponding to  definite  temperatures,  400°,  600°,  etc.,  of  the  thermo- 
couple when  the  resistances  R,  RI,  and  R2  are  properly  adjusted; 
variations  in  the  battery  a  are  controlled  by  the  standard  cell 
and  resistance  R.  This  arrangement  allows  calibrating  the  gal- 
vanometer immediately  before  each  experiment  and  give  the 
calibration  data  on  the  same  sheet  as  the  quenching  curve. 

The  string  galvanometer  had  a  resistance  of  6700  ohms;  its 
sensitiveness  may  be  adjusted  to  follow  that  of  the  thermocouple, 
although  this  is  not  necessary.  The  time  correction  of  this  gal- 
vanometer is  such,  fortunately,  that  the  directly  registered  curve 
is  simply  a  parallel  curve  to  the  one  which  would  be  obtained  if 
the  deflections  were  absolutely  instantaneous;  or  in  other  words, 
no  correction  for  the  inability  of  the  instrument  to  respond  in- 
stantaneously is  necessary.  There  remains,  of  course,  a  small 
unknown  time  correction  due  to  the  lag  of  the  thermocouple 
with  respect  to  the  test  specimen. 

In  Fig.  162  are  shown  curves  for  a  steel  of  0.42  carbon  quenched 
both  from  850°  and  950°  C.;  the  time  r  is  taken  to  100°  C.  The 
calibration  curves  are  also  shown  in  the  figure. 

Recording  Radiation  Pyrometers.  —  Any  phenomenon  whose 
magnitude  may  be  measured  by  the  deflection  of  a  galvanometer 
may  be  rendered  self-registering  by  optical  means.  Total  or 


424 


HIGH  TEMPERATURES 


\ 


\ 


\ 


\ 


\ 


$ 


3 

u 

bo 


I 

o> 


g 


RECORDING  PYROMETERS 


425 


monochromatic  radiation  falling  on  the  exposed  strip  of  a  bolom- 
eter may,  therefore,  be  made  to  record  its  intensity,  which  is,  as 
we  have  seen,  a  function  of  the  temperature  of  the  radiating 
body.  Langley,  in  1892,  rendered  his  bolometer  a  recording 
instrument,  the  records  being  taken  photographically.  This 
system  of  recording  has  been  used  mainly  for  the  mapping  of 
solar  spectra,  and  incidentally  for  the  estimation  of  the  sun's, 
temperature  and  in  other  astrophysical  investigations;  and  al- 
though it  might  be  used  in  laboratory  investigations  in  record- 
ing high  temperatures  in  terms  of  either  total  or  monochromatic 
radiation,  it  has  not  come  into  any  general  use  for  such  purposes. 
The  experimental  arrangements  are  necessarily  very  elaborate 


H 


5 


6         78         9        10       11    Noon     1         2         3         4         5        ft 
Fig.  163.     Solar  Radiation  Record. 

and  delicate,  for  descriptions  of  which  the  reader  should  consult 
the  Annals  of  the  Astrophysical  Observatory  of  the  Smithsonian 
Institution.  Callendar  has  applied  also  his  slide-wire  method 
of  recording  electrical  resistances  to  Langley's  bolometer.  The 
curve  of  Fig.  163  gives  the  record  of  solar  radiation  for  a 
day. 

The  radiation  pyrometers  of  the  Fery  type  are  readily  made 
self-registering,  it  being  only  necessary  to  substitute  for  the  indi- 
cating galvanometer  a  suitable  deflection-recording  instrument, 
such  as  the  Cambridge  thread  recorder  or  a  Siemens  and  Halske 
recording  milli voltmeter  of  the  required  range  and  sensitiveness. 
In  Fig.  164  is  shown  the  record  of  the  temperature  of  a  pottery 
"  biscuit  "  kiln  as  taken  with  a  Fery  radiation  pyrometer  and 
Cambridge  thread  recorder.  A  Callendar  slide-wire  recorder 


426 


HIGH  TEMPERATURES 


could  also  be  used  to  register  very  high  temperatures  if  too  great 
sensitiveness  be  not  demanded. 

The  Morse  or  Holborn  and  Kurlbaum  instruments  may  be 
made  semi-recording;  that  is,  a  registering  ammeter  may  be  put 
in  the  lamp  circuit  and  made  to  record  each  temperature  to  which 
the  pyrometer  is  set  by  the  observer.  This  method  would  have 
some  advantages  in  the  control  of  those  industrial  operations 
for  which  this  type  of  pyrometer  is  best  adapted. 


c 

1300 


nool 

1000' 

900C 
800C 
700 


1  8  9  10  11      Midnight      12345678 

Fig.  164.     Temperature  Record  of  Pottery  Kiln. 

Recording  Accessories.  —  We  may  mention,  finally,  a  number 
of  auxiliary  pieces  of  apparatus  and  methods  which  are  useful  in 
special  cases. 

Range  Control.  —  It  is  sometimes  desirable  to  limit  the  range 
of  the  recorder  to  some  restricted  temperature  interval  and 
thereby  gain  greater  sensibility  with  a  more  open  temperature 
scale.  This  may  be  done  in  several  ways.  We  shall  use  as 
illustrations  the  scale-control  box  of  Peake  as  applied  by 
the  Cambridge  Company  to  their  thread  recorder.  In  its 
more  complete  form  this  device  is  shown  in  Fig.  165,  for  use 
with  thermocouples  provided  with  Peake's  compensating  leads 
(page  176). 


RECORDING   PYROMETERS 


427 


A  6-volt  accumulator  passes  a  current  through  a  series  of 
fixed  resistances,  RZ,  RI,  R7,  and  a  portion  of  a  variable  resistance 
R2,  the  potentiometer  circuit. 

A  second  circuit,  the  pyrometer  circuit,  consisting  of  the  couple, 
leads,  R&  and  R5,  and  the  recorder,  is  connected  to  tap  onto  the 
ends  of  the  coil  R*  in  the  potentiometer  circuit.  Thus  the  poten- 
tial drop  in  RI,  due  to  the  current  from  the  accumulator,  is  opposed 
to  the  electromotive  force  of  the  couple,  and,  therefore,  at  some 
particular  temperature,  say  750°  C,  the  two  just  balance,  and 
no  current  will  flow  through  the  pyrometer  circuit.  If  now  the 


Accumulator 


hennocouple 


Switches  81  i  82  shown  in 

running  position;  dotted  lines 

show  test  petition 


Fig.  165.     Peake's  Scale  Control  Box. 

temperature  of  the  couple  falls,  a  current  will  flow  in  one  direction 
through  the  recorder,  whilst  if  it  rises  a  current  will  flow  in  the 
reverse  direction.  Thus  the  zero  or  undeflected  position  of  the 
recorder  pointer  may  be  made  in  the  center  of  the  scale,  and  will 
correspond  in  the  above  case  to  750°  C.,  whilst  the  resistance  R$ 
may  be  so  adjusted  that  one  end  of  the  scale  will  correspond  to 
600°  C.,  and  the  other  to  900°  C. 

The  accuracy  of  the  arrangement  depends  upon  the  current  in 
R*  being  maintained  constant,  and  to  secure  this  a  Clark  cell  is 
connected  across  the  resistances  R3  and  R*.  When  the  accumu- 
lator voltage  is  normal,  this  cell  does  not  give  any  current,  but  if 


428 


HIGH   TEMPERATURES 


accumulator  voltage  falls  slightly,  the  Clark  cell  gives  a  slight 
current  and  tends  to  keep  the  voltage  across  the  terminals  of  Rz 
and  RI  nearly  constant. 

The  change  in  E.M.F.  of  the  Clark  cell  with  temperature  also 
balances  very  nearly  the  corresponding  changes  in  the  compen- 
sating leads,  so  that  the  behavior  of  the  apparatus  is  nearly 
independent  of  cold-junction  temperature  fluctuations.  The 
arrangement  may  be  simplified  but  rendered  less  exact  by  dis- 
pensing with  the  Clark  cell. 


Automatic 
Commutator 


Pyrometer 
Galvanometer 


Hand 
Commutator 


Recording 
Galvanometer 


Fig.  1 66.     Recorder  and  Indicator  with  Four  Thermocouples. 

Multiple  Records  and  Circuits.  —  There  are  various  devices 
for  taking  several  records  on  a  single  sheet  by  means  of  one  gal- 
vanometer. They  practically  all  reduce  to  some  type  of  auto- 
matically driven  commutator  and  are  often  so  constructed  that 
the  several  records  may  be  distinguished  by  the  spacing  or 
length  of  dots  and  dashes.  While  it  is  proper  to  record  simul- 
taneously quite  different  temperatures,  it  is  usually  good  practice 
not  to  try  and  so  record  temperatures  that  frequently  overlap 


RECORDING   PYROMETERS 


429 


on  the  sheet,  as  its  interpretation  may  then  become  doubtful. 
It  is  also  often  convenient  to  have  on  one  circuit  a  recorder, 
which  may  be  in  an  office,  together  with  one  or  more  indicating 
instruments.  An  arrangement  of  four  thermocouple  circuits  is 
shown  in  Fig.  166,  whereby  a  recorder  gives  a  continuous  record 
for  all  the  couples,  and  the  temperature  reading  of  each  of  them 
may  also  be  taken  by  means  of  an  indicating  galvanometer. 


Thermometers 
Fig.  167.     Recorder  and  Indicator  with  Four  Resistance  Pyrometers. 


A  switchboard  may  also  be  used  permitting  interchangeability 
of  pyrometers,  as  illustrated  in  Fig.  167,  where  four  resistance- 
thermometer  circuits  are  provided  for,  to  be  used  with  various 
instruments  as  required.  See  also  Fig.  70. 

In  general,  it  may  be  said  that  almost  any  industrial  require- 
ment of  combination  of  pyrometer  circuits,  recording  and  indi- 
cating instruments  and  alarms  may  be  solved  satisfactorily  in 
practice. 


430  HIGH  TEMPERATURES 

Furnace  Control  and  Thermostats.  —  In  certain  operations, — for 
example,  in  taking  heating  and  cooling  curves,  —  it  is  advanta- 
geous to  raise  or  lower  the  temperature  of  the  electric  furnace 
continuously  at  a  uniform  rate.  This  is  readily  accomplished  for 
an  alternate-current  supply  by  the  use  of  a  salt-water  rheostat 
fed  on  heating  from  a  Mario tte  bottle.  The  metal  electrodes 
may  be  cut  to  shape  to  favor  the  uniformity  of  rise  in  tempera- 
ture; during  the  cooling  the  water  is  siphoned  off.  The  whole 
apparatus  may  be  made  completely  automatic,  if  desired,  so 
that  a  series  of  heating  and  cooling  curves  may  be  taken  at  any 
desired  rate  without  the  intervention  of  the  observer.  It  is  well 
to  keep  the  temperature  of  the  rheostat  down  by  circulating 
water  through  it  in  a  coil  of  pipe. 

It  is  often  desirable  to  maintain  a  furnace  at  constant  tem- 
perature. The  method  used  will  depend  largely  upon  the  tem- 
perature in  question. 

In  the  range  over  which  liquid  baths  may  be  used,  to  350°  C. 
with  suitable  oils,  they  are  satisfactory  when  properly  stirred 
and  provided  with  thermostatic  control.  With  a  sensitive  gas 
regulator  a  constancy  of  0.05°  C.  may  be  maintained,  and  with 
electric  control  a  somewhat  better  uniformity. 

A  uniform  temperature  over  a  large  volume  may  be  estab- 
lished by  means  of  a  vapor  in  equilibrium  with  its  liquid.  This 
system  is  not  available  for  high  temperatures,  and  it  is  difficult 
to  maintain  a  constant  temperature  over  long  periods  of  time. 

For  high  temperatures  air  baths  only  can  be  employed,  the 
most  usual  form  being  the  electric  resistance  tube  furnace. 
Special  windings  and  delicate  control  are  required  if  it  is  desired 
to  maintain  a  considerable  volume  at  constant  temperature. 
Various  devices  have  been  suggested  for  the  automatic  control 
of  furnace  temperatures,  based  usually  on  the  use  of  relays 
actuated  either  electrically  or  optically.  Most  of  the  recorders 
we  have  described  may  be  fitted  with  such  an  accessory. 

We  may  also  cite  the  optical  regulator  of  Kolowrat  which  will 
keep  an  electric  furnace  constant  to  2°  or  3°  at  1000°  C.,  and  may 
be  applied  to  either  thermoelectric  or  resistance  measurement  of 


RECORDING   PYROMETERS 


43  ! 


temperature.  The  light  from  a  powerful  source,  a  Nernst  lamp, 
is  reflected  from  the  galvanometer  mirror  onto  a  scale  repre- 
senting temperatures.  When  in 
adjustment  an  increase  in  tem- 
perature of  the  furnace  throws 
the  spot  of  light  onto  a  thermopile 
which  operates  a  series  of  relays 
cutting  in  resistance  to  the  heat- 
ing circuit  and  cooling  the  furnace 
slightly.  This  resistance  is  cut 
out  when  the  spot  leaves  the  ^ 
thermopile. 

Among  the  many  electric  ther- 
mostatic  controls  we  may  men- 
tion that  of  H.  Darwin,  which 
may  also  be  used  as  an  alarm 
(Fig.  1 68).  When  the  galvanom- 
eter needle  GV  is  deflected  from  the  stop  F,  due  to  a  rise  in 
temperature,  the  needle  engages  the  wheel  W  driven  by  clock- 
work and  makes  a  circuit  at  L,  which  may  be  either  that  of 
an  alarm,  as  shown,  or  that  of  a  regulating  circuit  by  means 
of  relays. 


Fig.  168. 


Darwin  Temperature 
Alarm. 


CHAPTER  XI. 
STANDARDIZATION   OF  PYROMETERS. 

Thermometric  Scales.  —  The  generally  recognized  standard 
temperature  scale  is  that  of  the  gas  thermometer,  which,  as  we 
have  seen,  has  been  realized  in  the  form  of  the  constant-volume 
nitrogen  thermometer  to  1550°  C.  This  scale  is  fixed  by  the 
determination  of  certain  reference  temperatures,  such  as  melting 
or  freezing  and  boiling  points.  It  would  be  desirable  to  define 
temperatures  in  terms  of  the  normal  or  thermodynamic  scale, 
which  is  independent  of  the  properties  of  any  particular  substance. 
At  the  present  time,  however,  the  limit  of  accuracy  attained  in 
gas  pyrometry  does  not  exceed  the  departure  of  the  constant- 
volume  gas  scale  from  the  thermodynamic  scale;  and  the  scale  as 
defined  by  various  gases  is  also  practically  identical,  so  that  for 
most  practical  purposes  we  may  speak  in  terms  of  either  scale 
interchangeably. 

Above  the  range  of  the  gas  thermometer,  we  are  compelled  to 
resort  to  extrapolation  in  terms  of  some  phenomenon  varying 
with  the  temperature.  For  this  purpose,  use  is  usually  made 
of  the  radiation  laws  based  on  the  relations  which  have  been 
found  to  exist  at  lower  temperatures  between  the  intensity  of 
total  and  monochromatic  radiation  and  temperature.  Just  as 
the  thermodynamic  scale  of  temperature  is  independent  of  the 
thermal  properties  of  any  particular  substance,  but  would  be 
reproduced  exactly  by  an  ideal  gas,  and  is  very  nearly  realized 
by  the  thermal  properties  of  ordinary  gases :  similarly,  the  radia- 
tion scale  of  temperature  is  independent  of  the  radiating  prop- 
erties of  any  particular  substance,  but  would  be  reproduced 
exactly  by  the  radiation  from  a  black  body,  and  is  very  nearly 
realized  by  the  radiation  from  an  almost  completely  closed,  clear 
furnace  at  a  uniform  temperature.  The  radiation  scale,  then, 

432 


STANDARDIZATION  OF  PYROMETERS  433 

may  be,  and  in  practice  is,  so  denned  as  to  be  the  thermodynamic 
scale,  so  that  we  have  in  reality  a  single,  continuous-temperature 
scale  from  the  lowest  to  the  highest  attainable  temperatures. 

Unfortunately,  there  is  not  as  yet  a  sufficiently  good  agree- 
ment among  the  few  temperatures  above  1200°  C.  determined 
with  the  gas  thermometer,  so  that  there  is  still  considerable 
uncertainty  in  the  values  to  assign  to  the  constants  in  the  radia- 
tion laws  and  therefore  to  fixed  points  in  the  higher  ranges. 

Fixed  Points.  —  As  the  scale  determined  by  the  gas  thermom- 
eter is  the  one  universally  recognized,  it  is  necessary,  in  order  to 
calibrate  a  pyrometer,  to  express  its  indications  in  terms  of  the 
gas  scale.  In  general,  it  is  not  feasible  to  compare  the  readings 
of  a  pyrometer  directly  with  those  of  the  gas  thermometer.  The 
use  of  the  latter  becomes  restricted  mainly  to  the  establishment 
of  certain  constant,  reproducible  temperatures  or  fixed  points 
such  as  are  given  by  freezing  points  and  boiling  points  of  the 
chemical  elements  and  of  certain  compounds.  The  accuracy  at- 
tainable in  pyrometric  researches  is,  therefore,  limited  by  the 
exactness  of  our  knowledge  of  these  reference  temperatures,  and 
their  determination  has  been  and  still  is  of  the  most  fundamental 
importance  in  pyrometry.  There  have  been  a  great  many  tem- 

jratures  suggested  for  this  use,  but  the  actual  number  available  is 
very  small.  Preference  should,  in  general,  be  given  to  those  deter- 
minations made  with  the  gas  thermometer  itself,  although  there 
are  others  made  indirectly  in  terms  of  the  gas  scale,  as  with 
thermocouples,  optical  pyrometers,  and  resistance  thermometers, 
which  are  of  considerable  weight;  and  in  fact  the  more  common 
practice,  when  working  with  the  gas  thermometer,  is  to  compare 
its  readings  in  a  furnace  or  bath  with  those  of  some  more  con- 
venient instrument  and  then  transfer  the  gas  scale  by  means  of 
the  latter  to  the  melting  or  boiling  points  by  interpolation. 

We  have  already  called  attention  to  many  of  these  determi- 
nations of  fixed  points,  among  which  the  following  may  be  con- 
sidered in  greater  detail: 

Sulphur. — (Boiling)  444.6°  C.  on  the  constant- volume  scale  of 
nitrogen,  or  444.5°  on  the  constant-pressure  scale;  correspond- 


434 


HIGH  TEMPERATURES 


ing  to  about  444.7°  on  the  thermodynamic  scale,  under  a  pressure 
of  760  mm.;  with  a  variation  of  0.090°  per  millimeter  change  of 
mercury  in  the  atmospheric  pressure. 

The  boiling  point  of  sulphur  has  been  the  object  of  several 
series  of  distinct  observations,  among  which  we  may  cite  the 
following,  distinguishing  between  direct  and  strictly  independent 
determinations  with  the  gas  thermometer  and  indirect  ones  by 
italicizing  the  former. 

BOILING  POINT  OF  SULPHUR. 

cop  Corr.  to 

Observers.  Method  and  remarks.  observed  const,  vol. 

/>o=i  at. 

Regnault Const,  vol.,  about 447-5° 

Crafts Const,  vol 445  .... 

Callendar  and 

Griffiths Const,  press 444 . 53        444 . 74* 

Reichsanstalt Const,  vol.  Wiebe  and  Botcher 

scale 444 . 5  444-5 

Chappuis  and  Harker .   Const,  vol.  corr.  from 445 . 2° .  .  444 . 7          444 . 7 

Holborn Const,   vol.   extrapolated   Pt. 

resistance 444-55        444-55 

Rothe Const,  vol.  Hg  thermo.  P.T.R. 

scale 444.7          444-8 

Thermocouples 445 .  o  .... 

Eumorfopoulos Const,  press 444-55        444-76 

Holborn  and  Henriing  Const,  vol 444-51        444-51 

Best  value  from  above  series .  .  .  .  444.  6 


Regnault's  figure  was  obtained  by  plunging  the  reservoir  of 
the  thermometer  in  the  liquid  sulphur;  but  this  liquid  will  super- 
heat, and  so  gives  too  high  a  value.  The  other  eight  very  con- 
cordant results  were  obtained  in  the  vapor. 

The  result  first  published  by  Chappuis  and  Harker,  using  a 
constant- volume  thermometer,  was  445.2°,  but  this  difference 
from  Callendar  and  Griffiths'  result  was  shown  probably  to  be 
due  mainly  to  an  incorrect  value  assumed  for  the  expansion  co- 
efficient of  the  porcelain  bulbs  used  by  the  former.  Eumorfo- 
poulos first  published  the  value  443.7°,  which  was  recognized  at 
the  time  to  be  uncertain,  as  it  depended  upon  the  unknown  ex- 
pansion coefficient  of  mercury,  which  has  since  been  determined 
by  Callendar  and  Moss  to  high  temperatures. 


STANDARDIZATION  OF  PYROMETERS  435 

Callendar  and  Griffiths  as  well  as  Eumorfopoulos  worked  with 
a  constant-pressure  air  thermometer,  and  it  is  of  interest  to  note 
that  the  outstanding  difference  between  several  of  the  experi- 
mental determinations  by  the  constant-volume  and  constant- 
pressure  methods  is  of  the  order  of  difference  to  be  expected 
between  the  two  gas  scales,  —  constant- volume  and  constant- 
pressure, —  as  seen  from  Callendar 's  table  (page  31)  and  from 
Fig.  i .  In  fact,  in  work  of  the  highest  precision  it  will  probably 
soon  be  desirable  to  reduce  observations  to  the  thermodynamic 
scale. 

At  the  Reichsanstalt  a  new  determination  of  the  S.B.P.  has 
recently  been  carried  out  by  Holborn  and  Henning,  using  sev- 
eral gases  and  bulbs  of  glass  and  quartz.  Their  result  is  about 
0.2°  C.  lower  than  would  be  expected  from  the  measurements  at 
constant  pressure. 

In  order  to  reproduce  the  exact  value  of  the  sulphur  boiling 
point,  however,  it  is  not  sufficient  to  plunge  the  protected  ther- 
mometer into  the  sulphur  vapor,  but  it  is  necessary  to  guard 
it  against  superheating  by  radiation  from  the  liquid  and  lower 
walls,  on  the  one  hand,  and  cooling  by  liquid  sulphur  condensed 
on  the  thermometer  case  and  radiation  from  the  thermometer, 
on  the  other  hand.  Unless  proper  precautions  are  taken,  varia- 
tions of  i°  C.  may  be  found.  Sulphur  boils  very  smoothly  with- 
out bumping,  and,  in  a  properly  constructed  apparatus,  condenses 
in  a  very  sharp  line  near  the  top  of  the  boiling  tube.  A  conical 
or  cylindrical  aluminium  shield  with  an  umbrella  cap  fitting  close 
about  the  thermometer  stem  serves  the  double  purpose  of  shield- 
ing the  instrument  from  radiation  and  condensed  sulphur.  A 
sulphur  boiling  apparatus  with  the  protected  thermometer  in 
place  is  shown  in  Fig.  169,  with  which  measurements  consistent 
to  about  0.03°  may  be  obtained.  Gas  or  electric  heating  may  be 
used,  and  the  boiling  tubes  may  be  of  hard  glass,  porcelain,  or 
aluminium.  A  study  by  Waidner  and  Burgess  of  the  various 
forms  of  sulphur  apparatus  used  by  previous  experimenters 
showed  that  they  give  the  same  temperature  to  a  few  hundred ths 
of  a  degree.  Commercial  sulphur  gives  the  same  boiling  point 


436 


HIGH  TEMPERATURES 


.Asbestos 


Aluminum 


ELECTRICALLY  HEATED  8.B.P.    APPARATUS 


GAS  HEATED  8.  B.  P.   APPARATUS 


Fig.  169.     Types  of  Sulphur  Boiling  Apparatus. 


STANDARDIZATION  OF  PYROMETERS  437 

as  *he  best  sulphur  obtainable.  A  criterium  of  satisfactory 
realization  of  the  S.B.P.  is  the  constancy  of  reading  when  a  ther- 
mometer with  accessories  is  displaced  several  centimeters  in  the 
vapor.  Waidner  and  Burgess  have  also  shown  that,  measured 
in  this  way,  the  column  of  vapor  above  boiling  sulphur  is  con- 
stant to  about  0.03°  C. 

In  spite  of  the  most  excellent  agreement  of  the  observations 
in  the  above  table,  the  determinations  with  the  hydrogen  ther- 
mometer by  Jaquerod  and  Wassmer  of  the  boiling  points  of 
naphthalene  and  benzophenone,  and  those  by  Day  and  Sosman 
with  the  nitrogen  thermometer  of  the  freezing  points  of  zinc 
and  cadmium,  are  not  consistent  with  the  value  cited  above 
for  sulphur.  As  shown  by  Waidner  and  Burgess,  using  the 
platinum  thermometer,  the  sulphur  point  as  quoted  would 
be  nearly  one  degree  too  high  in  terms  of  the  work  of  the 
observers  mentioned  above.  In  view  of  the  almost  universal 
use  of  the  sulphur  point  as  a  calibration  temperature,  it  is  of 
prime  importance  to  finally  fix  its  value  to  at  least  better  than 
0.1°  C. 

The  several  determinations  of  the  change  of  boiling  point  of 
sulphur  with  pressure  are  in  very  close  agreement.  For  exact 
work,  the  two- term  formula  of  Holborn  and  Henning,  or  that  of 
Harker  and  Sexton,  is  to  be  preferred. 

t  =  /?6o  +  0.0912  (H  —  760)  —  0.0442  (H  —  760) 2. 

Zinc.  —  (Freezing  or  melting)  419.4°  C.  Freezing  points  un- 
dergo unappreciable  changes  with  variations  in  atmospheric  pres- 
sure, and  their  experimental  determination  is  somewhat  easier 
than  for  boiling  points  if  a  thermocouple  is  used.  The  direct 
determination  of  a  metallic  freezing  or  melting  point  with  a  gas 
thermometer  is  beset  with  almost  insurmountable  experimental 
difficulties,  so  recourse  is  always  had  to  some  auxiliary  pyrometer 
whose  indications  have  been  exactly  calibrated  by  direct  com- 
parisons with  a  gas  thermometer. 

Zinc  is  easily  obtained  in  sufficient  purity.  Some  recent  de- 
terminations of  this  point  are: 


438  HIGH  TEMPERATURES 

Heycock  and  Neville 419.4°* 

Stansfield 418.2 

Holborn  and  Day 41  9 .  o 

Day  and  Sosman 418.2 

Waidner  and  Burgess 419 .37 

Holborn  and  Henning 4 19 .  40 

The  first  and  next  to  the  last  values  were  obtained  with 
the  resistance  pyrometer,  assuming  the  value  for  the  S.B.P., 
444.70°;  Stansfield's  observation  was  obtained  with  a  record- 
ing thermocouple,  and  the  other  is  by  direct  transfer  with 
thermocouples  or  resistance  thermometers  from  the  nitrogen- 
gas  thermometer. 

Zinc. — (Boiling)  920°  C.,  with  a  variation  of  0.15°  for  a  change 
of  i  mm.  in  the  atmospheric  pressure. 

The  boiling  point  of  zinc  has  been  the  object  of  a  great  many 
determinations,  and  yet  it  is  one  of  the  least  known  and  con- 
sequently the  most  unreliable  to  try  to  use,  and  is  not  to  be 
recommended.  It  has  been  the  object  of  so  much  study,  un- 
doubtedly, as  it  was  apparently  the  one  point  near  the  upper 
limit  of  the  early  experiments  with  the  gas  thermometer  which 
could  be  determined  directly  by  this  instrument;  but  super- 
heating effects  in  vapors  at  such  high  temperatures  and  an  un- 
even temperature  distribution  are  very  difficult  to  obviate  even 
with  electrical  heating. 

Some  of  the  results  obtained  are  shown  by  the  following  table : 

E.  Becquerel 930°  and  890°  C. 

Sainte-Claire-Deville 915    to     945 

Barus 926    and  931 

Violle .  930 

Holborn  and  Day  (two  observations) 910    and  930 

Callendar 916 

D.  Berthelot 918 

The  value  930°  as  given  by  Voille's  and  Barus'  results  was 
generally  accepted  until  recently,  but  the  more  recent  deter- 
minations indicate  930  to  be  over  10°  high.  The  value  adopted, 
920°,  is  probably  not  in  error  by  over  5°  C. 

*  The  value  419.0°  is  obtained  if  an  observation  on  an  admittedly  too 
small  sample  be  included. 


STANDARDIZATION  OF   PYROMETERS  439 

Gold.  —  (Fusion  or  freezing)  io6f  C.  This  point  is  to-day 
one  of  the  best-known  fixed  points,  and  gold  possesses  the  ad- 
vantages of  being  obtainable  in  very  great  purity,  is  not  oxidiz- 
able  in  air,  nor  is  it  readily  attacked  by  the  silicious  materials 
used  in  crucibles.  Its  cost  is  its  only  drawback  for  use  in  con- 
siderable quantities,  but  methods  have  been  devised,  as  insert- 
ing a  short  length  of  wire  between  the  leads  of  a  thermocouple, 
requiring  only  very  minute  amounts  of  gold.  These  wire  methods 
give  on  the  average  the  same  results  as  the  crucible  method,  as 
shown  by  Holborn  and  Day  and  by  D.  Berthelot,  although  their 
precision  is  slightly  less. 

The  early  determinations  of  the  gold  point  were  quite  dis- 
cordant, but  the  later  ones  where  electric  heating  was  employed 
are  in  excellent  agreement. 

Pouillet 1180°  C. 

E.  Becquerel 1092  and  1037 

Violle 1045 

Holborn  and  Wien 1070  to     7075 

Heycock  and  Neville 1062 

D.  Berthelot 1064 

Holborn  and  Day 1064 

Jaquerod  and  Perrot 1067 

Day  and  Sosman. 1062 

Violle 's  value  was  long  quoted  as  the  best  for  the  gold  point, 
but  the  later  determinations  show  it  to  be  some  20°  low.  Hol- 
born and  Wien's  high  value  was  obtained  with  a  porcelain-bulb 
thermometer  and  is  to  be  considered  as  replaced  by  Holborn  and 
Day's  value,  to  obtain  which  nitrogen  in  a  Pt-Ir  bulb  was  used, 
together  with  a  thermocouple.  The  agreement  of  their  results 
when  working  under  various  conditions  is  shown  from  the  follow- 
ing observations: 

Gold,  sample  i 1064.0  ±  0.6 (crucible  method) 

Gold,  sample  2 1063 . 5  (crucible  method) 

Gold,  sample  2 1063 .9  (wire  method) 

Not  less  than  300  grams  was  used  for  observations  in  both 
graphite  and  porcelain  crucibles,  while  by  the  wire  method  0.03 
gram  of  the  metal  suffices. 

Berthelot  used  his  optical  gas  pyrometer  in  connection  with 


440  HIGH  TEMPERATURES 

thermocouples  and  considers  his  result  to  be  in  error  by  less  than 
2  degrees.  Heycock  and  Neville's  result  was  obtained  by  extrap- 
olation above  the  sulphur  point  of  the  platinum-resistance  for- 
mula, while  Jaquerod  and  Perrot's  value  was  obtained  in  terms 
of  a  quartz-bulb  constant-  volume  thermometer  filled  with  various 
gases,  the  results  agreeing  to  a  few  tenths  of  a  degree.  They 
used  a  modified  form  of  the  wire  method,  which  consisted  in 
making  a  small  piece  of  gold  wire  a  part  of  an  alternating  electric 
circuit,  melting  of  the  gold  being  noted  by  cessation  of  sound  in 
a  telephone. 

Day  and  Sosman  used  their  nitrogen  thermometer  previously 
described.  A  preliminary  determination  with  the  same  appa- 
ratus by  Day  and  Clement  gave  1059°  for  the  gold  point  on 
a  sample  found  subsequently  to  contain  iron.  It  was  unfor- 
tunate that  Holborn  and  Valentiner,  in  their  gas-thermometer 
work  to  1600°  C.,  did  not  repeat  the  gold  point.  An  examina- 
tion of  their  thermoelectric  data  shows  a  discrepancy  of  about 
5  degrees  at  this  temperature  from  the  value  here  cited  as  most 
probable. 

Berthelot  has  called  attention  to  the  fact  that  the  later  deter- 
minations are  sufficiently  concordant  to  warrant  reducing  them 
to  the  thermodynamic  scale  (see  page  26). 


Observer,  Gas.  Corrections.     Observed 


D.  Berthelot  ...............       Air          76  cm.  +  i  .36°C.    1064°       1065.6° 

Holborn  and  Day  ..........         N  29  cm.      0.27          1064         1064.3 

Jaquerod  and  Ferret  .......  j  OCO  \  23  cm'      °'21          1067.2     1067.4 

Day  and  Sosman  ...........         N          21  cm.      0.21          1062.4     1062.6 

Silver.  —  (Freezing  or  melting)  961.0°.  The  freezing  point  of 
silver  is  not  a  constant  temperature  except  in  a  reducing  atmos- 
phere, and  this  metal  is  volatile,  thus  making  it  unsafe  to  use 
under  conditions  in  which  its  vapors  may  attack  platinum  wires, 
as  of  a  thermocouple  whose  electric  properties  silver  alters  very 
considerably. 

Many  determinations  of  this  point  have  been  made,  but  it  is 
only  the  recent  observations  that  take  into  account"  the  effects 


STANDARDIZATION  OF   PYROMETERS  441 

of  oxidizing  and  reducing  atmospheres.     Some  of  the  determi- 
nations of  the  silver  point  follow: 

Pure  Ag.  In  air. 

Pouillet I000°  c. 

E.  Becquerel 060  and  016 

Violle gj4 

Holborn  and  Wien 070 

Heycock  and  Neville 060  <  occ 

D.  Berthelot 962  957 

Holborn  and  Day 961 .5  955 

Day  and  Sosman .         960.0 

Waidner  and  Burgess 960.9  953  to  957 

Melted  silver  exposed  to  the  air  gradually  absorbs  oxygen, 
which  lowers  the  freezing  point,  and  this  latter  is  not  a  definite 
temperature,  varying  with  the  rate  of  cooling,  mass,  and  sur- 
roundings. This  lowering  may  reach  20  degrees  or  more.  The 
wire  method  gave  953 .6  =t  0.9  as  found  by  Holborn  and  Day.  The 
freezing  point  of  pure  silver  may  be  obtained  in  a  graphite  crucible 
in  an  atmosphere  of  nitrogen  or  of  CO,  or  covered  with  powdered 
graphite,  i.e.,  in  conditions  preventing  oxidation.  The  melting 
or  freezing  point  is  equally  sharp,  and  on  account  of  the  ease 
of  getting  very  pure  silver  its  use  is  strongly  recommended  as  a 
fixed  point. 

Copper.  —  (Freezing  or  melting)  io6f  in  air,  io8f  pure. 
Whether  the  gold  or  the  copper  point  was  the  higher  was  long 
an  open  question  in  pyrometry.  The  great  advantage  in  prac- 
tice of  copper  is  its  cheapness,  but  the  fact  that  copper  appar- 
ently has  two  freezing  points  does  not  possess  the  same  disad- 
vantages as  with  silver,  for  both  of  the  copper  points  are  very 
definite,  the  higher  one,  1083°,  being  that  of  the  pure  metal,  easiest 
obtained  with  a  graphite  crucible,  the  metal  being  protected 
from  the  air  by  a  layer  of  powdered  graphite.  The  lower  value, 
1063°,  is  given  by  the  wire  method,  and  copper  may  replace  gold 
in  this  way.  Values  intermediate  between  1063°  and  1083°  will 
be  obtained  in  crucibles  for  incomplete  protection  from  air,  the 
effect  being  due  to  the  formation  and  solution  of  cuprous  oxide, 
saturation  of  the  copper  with  the  oxide  giving  the  eutectic  point 
1063°  for  about  3.5  per  cent  CuzO.  The  presence  of  the  eutectic 
temperature  will  usually  be  detectable,  whatever  the  percentage 


442  HIGH  TEMPERATURES 

of  Cu2O  present,  and  this  fact  may  be  used  to  check  the  purity 
of  copper  in  a  crucible. 

We  may  note  the  following  determinations  of  the  copper  point: 

Heycock  and  Neville 1080 . 5° 

Stansfield 1083 

Holman Io86 

Holborn  and  Day 1084.1 

Day  and  Sosman 1082.6 

Waidner  and  Burgess. 1083 

The  values  obtained  by  Holborn  and  Day,  and  by  Day  and 
Sosman,  are  the  only  ones  determined  directly  in  terms  of  the 
gas  thermometer.  The  difference  of  20°  C.  between  the  Cu  and 
Cu-Cu2O  points  has  been  determined  by  various  observers. 

Palladium.  —  (Fusion)  1550°.  This  temperature  marks  the 
present  upper  limit  of  the  gas  thermometer.  The  following 
are  some  of  the  recent  determinations  of  the  palladium  melting 
point: 

Observed  p -.}„_-,»*» 
Observers.  Method.  melting    ™5J5«? 

Nernst  and  v.  Warten- 

berg Optical;  Wien's  law,  c2  =  14, 600  1541°          1546° 

Waidner  and  Burgess  Optical;  Wien's  law,  c2=  14, 500  1546             1546 

Holborn  and  Valenti-  Nitrogen    gas,    thermocouples, 

ner and  optical c2=i4,2Oo  1575  T560 

Day  and  Sosman Nitrogen  gas  and  thermocouples  1549 

Palladium  may  be  melted  in  air  by  the  wire  method  and  there- 
fore is  a  convenient  control  temperature  for  thermocouples. 
(See  p.  186.)  It  will  be  noted  that  the  gas- thermometer  deter- 
mination of  Day  and  Sosman  appears  to  be  equivalent  to  a 
value  of  £2  =  14,450  in  the  Wien  equation. 

Platinum.  —  (Fusion)  1755°.  Above  the  palladium  point, 
resort  must  be  had  to  extrapolation.  There  have  been  a  great 
many  experimental  estimates  made  of  the  platinum  melting 
point,  some  of  them  based  only  on  extrapolation  of  purely 
empirical  formulae  from  temperatures  below  1100°  C.  Such,  for 
instance,  are  the  thermoelectric  estimates  based  on  the  formula 
E  =  —  a  -j-  bt  +  ct*  on  data  which  satisfy  this  equation  only  in 
the  range  300°  to  1200°  C.  In  view  of  the  great  importance  of 
this  temperature  as  the  best  one  for  reference  in  the  upper  part 


STANDARDIZATION  OF  PYROMETERS 


443 


of  the  scale,  all  the  determinations  of  which  we  are  aware  that 
have  been  made  are  included  in  the  table.  The  values  found 
prior  to  the  year  1900  are  in  terms  of  incorrect  values  of  the 
basal  temperatures,  and  cannot  therefore  be  correct  except  by 
accident. 

EXPERIMENTAL  DETERMINATIONS  OF  THE  MELTING  POINT 


OF  PLATINUM. 

Published 

Reduced 

Date.               Observers. 

Method. 

melting 

to  common 

point. 

scale.  * 

1877-1879  Violle 

Calorimetric 

^775-1779 

1892       Barus 

Thermoelectric 

1757-1855 

Q        j  Holborn  and 
1895    }    Wien 

>  Thermoelectric 

1780 

o  6    {  Holman,  Law- 
9      {    rence  and  Barr 

|  Thermoelectric 

1760 

1898      Petavel 

Total  light  from  Pt 

1766 

1903       Nernst 

j  Total  light  from  black 
(    body 

\    1782 

(  Holborn  and 

Thermoelectric  and  opti- 

j 

1905    \    Henning 

cal 

(     1729 

.    j  Holborn  and 
I9°5    i    Henning 

|  Thermoelectric 

1710 

1755 

1905      Harker 

Thermoelectric 

1710 

J755 

^    (  Nernst  and 
1900    )    Wartenberg 

Optical;  Wien's  law 
(c2=i4,6oo) 

|     1745 

I75i 

(  Waidner  and 

Optical;  Wien's  law 

j 

I7"?3 

1907    i    Burgess 

(     1753 

(  Holborn  and 
1907    i    Valentiner 

Optical;  Wien's  law 
(£2=14,200) 

j     1782 

1763 

(  Waidner  and 

Monochromatic  radiation 

j 

1907    i    Burgess 

from  Pt 

(     1750 

1750 

(  Waidner  and 
1907    i    Burgess 

Thermoelectric                    1 
(two  formulae)                    j 

1706-1730 

1753 

f  Monochromatic  radiation 

T7>  ' 

I      from  Pt: 

1909       Fery 

1  Oxidizing  atmos. 

1690 



I  Reducing  atmos. 

1740 

.... 

1910      Sosman 

j  Thermoelectric  from 
I    Pd=i549° 

1752 

1755 

1910      Ruff 

Optical                               about  1750 

Best  Value  



1755 

*  This  scale  is  that  for  which  c^= 14,500  in  Wien's  law  III,  p.  251. 

The  published  thermoelectric  determinations  involving  extrap- 
olation on  the  thermoelectric  scale  (equation  (3) ,  page  112)  from 
low  temperatures  have  little  or  no  weight.  The  method  used 
by  Nernst  in  1903  is  not  capable  of  great  accuracy.  Fery's  as 
well  as  Ruff's  measurements  appear  to  have  been  crude,  and  the 
differences  noted  by  the  former  may  be  due  to  the  surface  prop- 


444  HJGH  TEMPERATURES 

erties  of  the  platinum  in  the  different  parts  of  a  gas  flame  and 
not  to  the  oxidizing  and  reducing  atmospheres  as  such.  All  the 
optical  measurements  by  the  other  observers  were  taken  in  an  ox- 
idizing atmosphere  and  are  at  least  50  degrees  higher  than  Fery's 
oxidizing- atmosphere  values.  The  outstanding  uncertainty  of 
the  platinum  point  is  mainly  attributable  to  the  difference  as- 
signed to  Cz  in  the  Wien  formula  and  to  the  different  gas  scales 
in  terms  of  which  the  extrapolations  are  made.  The  value  here 
assigned  to  the  platinum  point,  1755,  is  in  terms  of  the  Day  and 
Sosman  gas  scale  (Pd  =  1549),  the  optical  determinations  of 
Nernst  and  v.  Wartenburg,  and  Waidner  and  Burgess,  and  the 
mean  differences  between  the  palladium  and  platinum  points  as 
found  by  them  and  by  Holborn  and  Valentiner,  thus: 

Observers.  Pt-Pd. 

Nernst  and  v.  Wartenberg 204°  C. 

Holborn  and  Valentiner 207 

Waidner  and  Burgess 207 

Rhodium.  —  (Fusion)  1940°.  The  other  members  of  the  plat- 
inum group  have  had  their  melting  points  less  well  determined 
than  palladium  and  platinum.  For  rhodium  the  following  esti- 
mates, among  others,  have  been  made: 

Mendenhall  and  Ingersoll  (Pt=  1755) 1932 

V.  Wartenberg  (using  a  tungsten  furnace) 1940 

Iridium.  —  (Fusion)  2300°.  Although  it  is  questionable  if 
temperature  of  2000°  C.  and  over  can  be  determined  in  terms 
of  the  gas  scale,  it  may,  nevertheless,  be  found  desirable  to  deter- 
mine as  exactly  as  may  be  one  or  more  fixed  temperatures  in 
this  range  by  other  methods,  as  specific  heat  and  the  laws  of 
radiation.  Iridium  and  tungsten  seem  to  be  the  most  suitable 
for  this  purpose.  Hardly  any  limit  of  accuracy  can  as  yet  be 
placed  upon  such  determinations.  For  iridium  the  following 
values  have  been  found: 

Violle 1950°  C. 

Veder  Weyde 2200 

Nernst 2200  to  2240 

Rasch  (computed  from  Nernst' s  data) 2285 

Mendenhall  and  Ingersoll 2300 

v.  Wartenberg 2360 


STANDARDIZATION  OF  PYROMETERS 


445 


V.  Wartenberg's  determination  was  made  in  a  tungsten  furnace 
in  vacuo;  that  of  Mendenhall  and  Ingersoll  of  a  bead  on  a  Nernst 
glower. 

The  recent  development  of  furnaces  suitable  for  use  at  these 
extreme  temperatures  will  undoubtedly  enable  us  to  more  sharply 
define  other  points  in  this  part  of  the  scale. 

It  is  interesting  to  note  that  at  the  extreme  temperature  of 
the  electric  arc,  3600°  C.,  the  various  radiation  methods  and  the 
specific-heat  method  give  results  agreeing  to  about  100°  C. 

Other  metals  melting  below  1100°  C.,  such  as  cadmium,  lead, 
antimony,  and  aluminium,  have  also  been  used  in  the  attempt  to 
determine  fixed  points,  and  some  of  the  results  are  given  in  the 
accompanying  table  for  metals  melting  below  1100°  C. 

TABLE  OF  FREEZING  POINTS  TO  1100°  C. 


Observers...  .  \ 
Date  

Stansfield. 
1898 

D.  Berthe- 
lot. 

1898-1901 

Heycock  and 
Neville. 
Callendar. 
1895-99 

Waidner 
and  Burgess 

1910 

Holborn 
and  Day. 

1900-1901 

Day  and 
Sosman. 

1910 

Instrument.  .  . 

Recording 
thermo- 
couple. 

Optical 
interference. 

Electrical 
resistance. 

Electrical 
resistance. 

Nitrogen 
ther.  and 
thermo- 
couple. 

Nitrogen 
ther.  and 
thermo- 
couple. 

Calibration     ( 
data  j 

0° 
IOO 

444-53 

Expansion 
of  air. 

0° 
IOO 

444.53 

0° 
IOO 

444-70 

Pt-Ir-bulb 
nitrogen 
thermometer 

Pt-Rh-bulb 
nitrogen 
thermometer 

Sn 

2*2    1° 

2^1.9° 

2^1.9° 

Bi 

268  4 

269    2 

Cd  

320.  7 

321  .0 

321  .  7° 

320   0° 

Pb  

32S  .O 

327.  7 

327.4 

326  .9 

Zn  
Sb  
Al  

418.2 
640    2 

419.0 
629.5 

645.5 

419.4 
630.7 
658.0 

419.0 
630.6 
657.0 

418.2 
629.2 
658.0 

Ag3~  Cll2 

778  s 

779-  2 

Ag  (in  air) 

nee   o 

9[>t?  -O 

Ag  (pure).. 
Au  

961.5 

1062  7 

962 
1064 

960.7 

1061  .  7 

960.9 

961.5 
1064  .  o 

960.0 
IO62  .4 

Cu-Cu2O. 

1063  .  2 

1064  .  9 

Cu  

1083  .  o 

1080  .  5 

1083.0 

1084  .  i 

1082.6 

The  Iron  Group.  —  A  fixed  point  that  has  been  frequently 
used  is  the  melting  point  of  nickel  (1450°).  The  thermoelectric 
determinations  based  on  empirical  formulae  gave  values  varying 


446  HIGH  TEMPERATURES 

from  1484  to  1427.  Day  and  Sosman  find  1452  with  the  gas 
thermometer,  and  Ruer  1451  with  the  thermocouple,  assuming 
palladium  =  154 1.  The  former  find  for  cobalt  1490,  and,  from 
the  measurements  of  several  observers,  iron  would  have  a  melt- 
ing point  of  about  1520  on  the  same  scale.  These  metals  are 
readily  oxidized  and  usually  contain  sufficient  impurities  to  in- 
fluence their  melting  temperatures  somewhat.  They  are  most 
readily  worked  in  an  atmosphere  of  hydrogen.  Microscopic 
samples  melted  on  platinum  in  hydrogen,  as  measured  by  Burgess 
with  an  optical  pyrometer  (see  p.  343) ,  gave  Ni  =  1435,  Co  =  1464, 
and  Fe  =  1505. 

Metals  Melting  above  2000°  C.  —  Above  the  platinum  point, 
there  have  been  recently  a  considerable  number  of  attempts  to 
locate  fixed  points.  With  the  exception  of  iridium  and  rhodium, 
which  have  already  been  mentioned,  it  appears  to  be  necessary 
to  work  in  vacuo  all  the  elements  available  in  this  very  high 
temperature  region.  A  very  convenient  way  to  mount  them  is  as 
filaments  or  strips  as  in  incandescent  lamps. 

Of  the  elements  that  have  been  so  studied,  only  tantalum 
and  tungsten  have  been  determined  with  a  fair  agreement  by 
several  observers;  and  tungsten  is  the  only  one  which  melts 
without  excessive  evaporation,  and,  having  the  highest  melting 
point  yet  measured,  appears  to  be  the  best  adapted  for  an  extreme 
fixed  point. 

For  tungsten  the  following  values  have  been  found: 

"Waidner  and  Burgess  (1906-1910)  (£2=14,500). . .  .   3250-3050°  C. 

V.  Wartenberg  (1907-1910)  (^2=14,600) 2800-2900 

Pirani  (1910)  (c2  =  14.500) 3250 

The  value  3000°  C.  is  probably  correct  to  100°  C. 
For  tantalum  we  have: 

V.  Bolton  (1905) 2250-2300° 

Waidner  and  Burgess  (1907  and  1910) 2910 

Pirani  (1910) 3000 

Pirani  and  Mayer  (1911) 2850 

Measurements  of  the  melting  points  of  osmium,  molybdenum, 
titanium,  and  other  very  refractory  elements  have  also  been 


STANDARDIZATION  OF  PYROMETERS 


447 


made,  but  none  of  them  gives  promise  of  being  as  serviceable  as 
the  above  for  fixed  points  in  pyrometry. 

Melting  Points  of  the  Chemical  Elements.  —  In  Table  II  of  the 
Appendix  is  given  a  list  of  the  melting  points  for  the  chemical 
elements  with  some  indication  of  our  knowledge  of  their  exactness. 


Fig.  170.     Freezing  of  Copper. 

Typical  Freezing-point  Curves.  —  The  freezing-point  curves  of 
copper,  antimony,  silver,  and  aluminium  are  shown  in  Figs.  170 
to  173,  from  data  obtained  at  the  Bureau  of  Standards,  in  which 
time  in  minutes  is  plotted  as  abscissa  and  E.M.F.  of  a  90  Pt-io  Rh 


448 


HIGH   TEMPERATURES 


.10 
47.000 
.900 

I- 

\ 

\ 

> 

1787  A    P.P.  Sb 
F.P.=46.8367o>o630°7  C. 

\ 

\ 

A 


\ 

7^ 

4= 

=^f- 

=*= 

=F= 

=*= 

=F^ 

[ 

F= 



•H  , 

->• 

--^ 

%  Resistance  in 

l_i__i_JL_J 

\ 

1 

\ 

\ 

\ 

1 

\ 

1 

\ 

\ 

\ 

V 

\ 

\ 

V 

619^6  C. 

1H.18'20    22    24    26    28     30    32    34    36    38    40    42    44    46    48     50    52    54    56    58  12H.O'02  04    06 

Time  in  Minutes 

Fig.  171.     Freezing  of  Antimony. 


21W.98 


#1787F    P.P.  Ag 
F.P.=12.1622O960?72  C. 


*478F     F.P.Ag 
F.P.=21.8902«.960°75  C. 


3  hrs.  02  m.  04    06    08    10    12    It    16    18    20    22    24    26    28    30 


Fig.  172.     Freezing  of  Silver. 


STANDARDIZATION  OF  PYROMETERS 


449 


thermocouple  as  ordinate  for  copper  and  aluminium,  and  the 
resistances  of  a  platinum  thermometer  for  antimony  and  silver. 
An  inspection  of  the  copper  curve  shows  why  this  metal  is  desir- 
able to  use,  as  it  gives  a  very  flat  curve.  With  aluminium  rapid 


Fig.  173.     Freezing  and  Melting  of  Aluminium. 

cooling  would  be  fatal  to  an  exact  determination.  The  slant 
here  observed  in  the  curve  at  the  transition  point  is  characteristic 
of  the  presence  of  impurities  and  of  low  conductivity  and  latent 
heat.  For  this  metal  the  melting  curve  is  also  given,  showing 
the  melting  and  freezing  points  to  differ  somewhat,  apparently. 


450  HIGH  TEMPERATURES 

Antimony  undergoes  great  undercooling,  depending  on  the  rate 
of  cooling,  and  may  reach  over  30°  C.;  but  the  maximum  is  a  very 
definite  temperature  for  moderate  undercooling,  and  for  a  quick- 
acting  thermometer  in  a  charge  of  metal  that  is  not  too  small. 
For  silver,  two  rates  of  cooling  are  shown. 

Boiling  Points.  —  Sometimes  it  is  desired  to  calibrate  a  pyrom- 
eter down  to  room  temperature,  even  if  in  this  case  the  use  of 
a  mercury  thermometer  is  usually  to  be  preferred.  Use  may  be 
made  of  the  boiling  points  of  water,  aniline  or  naphthaline,  and 
benzophenone,  or  of  the  tin  freezing  point,  231.9°. 

Water.  —  100°  by  definition,  with  a  variation  of  0.04°  for  a. 
change  of  i  mm.  in  atmospheric  pressure. 

Aniline.  —  184.1°,  with  a  change  of  0.05°  per  millimeter.  This 
value  is  probably  correct  to  0.1°.  Aniline,  however,  oxidizes 
readily. 

Naphthalene.  —  218.0°,  with  a  change  of  0.058°  per  millimeter. 
This  point  has  been  very  carefully  determined  by  several  ob- 
servers (see  page  226),  and  naphthalene  is  cheap  and  readily 
obtained  of  sufficient  purity,  best  tested  by  taking  its  freezing 
point,  which  should  be  80.0°  C. 

Benzophenone. — 306.0°,  with  a  change  of  0.063°  Per  millimeter. 
Although  expensive  and  difficult  to  get  pure  (melting  point  = 
47.2°),  this  substance  appears  to  be  the  only  satisfactory  one 
so  far  found  possessing  a  sufficiently  constant  boiling  point 
between  218°  and  445°.  The  sulphur  boiling-point  apparatus 
(Fig.  169)  may  be  used  for  both  naphthalene  and  benzophenone 
if  provided  with  an  auxiliary  condensing  tube.  Both  these  boil- 
ing points  are  easily  kept  constant  to  better  than  0.05°. 

Metallic  Salts.  —  The  different  fixed  points  that  have  been 
mentioned  are  not  all  of  a  very  convenient  use.  It  would  be 
preferable  to  have  in  the  place  of  the  metals,  metallic  salts  for 
the  determination  of  the  fixed  points  if  they  can  be  shown  to 
be  satisfactory  otherwise.  These  salts  fortunately  are  for  the 
most  part  without  action  on  platinum,  which  is  of  great  advan- 
tage for  the  standardization  of  thermocouples  and  resistance 
thermometers.  There  are  few,  however,  whose  fusing  points, 


STANDARDIZATION  OF  PYROMETERS  451 

have  been  determined  up  to  the  present  time  in  a  sufficiently 
precise  manner. 

Among  the  salts  whose  freezing  or  melting  temperatures  have 
been  carefully  determined,  and  which  may  therefore  be  used  for 
calibration  purposes,  are: 

NaCl  (melting)  by  W.  P.  White 801°  C. 

NaCl  (freezing)  by  G.  K.  Burgess 800 

Na2SO4  (melting)  by  W.  P.  White 885 

Diopside  (CaMg(SiO3)2)  (melting)  by  Day  and  Sos- 

man 1391 

Anorthite  (CaAl2Si2O8)  (melting)  by  Day  and  Sosman  1549 

Lithium  metasilicate  (Li2SiO3)  (melting),  F.  M.  Jaeger  1202 

Sodium  metasilicate  (Na2SiO3)  (melting),  F.  M    Jaeger  1088 

The  transformation  points  of  some  salts  can  only  be  obtained 
satisfactorily  on  heating  due  to  great  undercooling  when  it  is 
attempted  to  take  their  freezing  points.  This  is  true,  for  in- 
stance, of  diopside,  anorthite,  and  the  silicates,  the  values  for 
which  apply  only  for  chemically  pure  salts  prepared  artificially. 
Where  stirring  is  practicable  the  undercooling  can  largely  be 
avoided  in  taking  freezing  points  of  both  metals  and  salts. 

In  general,  salts  give  a  less  sharp  melting  point  than  metals, 
due  mainly  to  low  conductivity  and  heat  of  fusion  of  the  former, 
and  of  course  impurities  will  act  in  the  same  way.  There  is  no 
difficulty,  however,  in  keeping  the  melting-point  curve  flat  to 
within  i°  C.  for  some  pure  salts  such  as  NaCl  and  N2SO4.  There 
are  undoubtedly  other  salts  which  might  be  studied  to  advantage, 
such  as: 

Melting  point. 

i  MolNaCl+i  MolKCl about    650° 

Pb2O5  2  Na2O about  1000 

MgSO4 about  1150 

K2S04  has  been  used  to  some  extent,  but  it  appears  to  possess 
several  dimorphous  varieties  with  different  melting  points,  like 
sulphur,  so  that  the  actual  point  observed  may  be  uncertain. 
On  page  367  is  given  a  list  of  salts  and  the  status  of  their  melt- 
ing points  as  determined  by  relatively  less  precise  methods  than 
the  above. 

Alloys :  Eutectic  Points.  —  In  the  case  of  certain  alloys  there 
are  well-defined  transition  points  which  may  be  used  as  fixed 


452  HIGH   TEMPERATURES 

temperatures  to  advantage  in  those  temperature  intervals  in 
which  there  is  no  conveniently  located  and  suitable  metal  freezing 
point.  The  most  sharply  denned  of  such  transformations  are  the 
temperatures  of  freezing  of'eutectics,  when,  if  the  components 
are  pure  and  the  alloy  is  of  very  nearly  the  eutectic  composition, 
the  evolution  of  heat  and  the  constancy  of  temperature  during 
the  transformation  compare  favorably,  in  some  cases,  with  the 
freezing  of  a  pure  metal. 

Such  a  suitable  eutectic  in  a  desirable  location  is  that  of  sil- 
ver and  copper,  which  happens  to  have  the  composition  Ag3-Cu2, 
whose  freezing  temperature  has  been  found  by  Heycock  and 
Neville  to  be  779.0°,  and  by  Waidner  and  Burgess  779.2°.  There 
are  probably  a  considerable  number  of  such  transformation 
temperatures  that  could  be  used  as  fixed  points  to  advantage. 
Thus  the  eutectics  of  aluminium  or  of  antimony  with  the 
members  of  the  iron  group  are  probably  more  sharply  defined 
temperatures  than  those  of  the  commercial  metals  often  used 
in  standardizing  thermocouples.  Another  well-known  and  fairly 
reproducible  transformation  temperature  on  cooling  in  the  solid 
state  is  the  recalescent  point  of  steel  (iron-carbon),  of  maximum 
effect  for  C  =  0.9%  at  about  705°  C.  for  slow  cooling  and  some- 
what lower  for  fast  cooling. 

Reproducibility  of  Freezing  Points.  —  It  is  of  great  importance 
in  pyrometry  of  precision  and  in  the  calibration  of  instruments 
to  be  able  to  reproduce  exactly  the  fixed  temperatures  of  boiling 
and  freezing  or  fusion.  We  have  seen  that  the  materials  ordi- 
narily used  for  boiling  points  can  easily  be  had  in  sufficient 
purity  to  reproduce  these  temperatures  to  within  0.05°,  and  that 
their  freezing  points  are  a  delicate  test  of  purity. 

There  have  been  several  intercomparisons  of  the  thermal  repro- 
ducibility  of  some  of  the  metals  whose  freezing  temperatures 
are  used  as  fixed  points.  Thus,  Day  and  Allen  in  1904,  using 
thermocouples,  found  that  the  metals  used  in  the  establishment 
of  the  Reichsanstalt  scale  could  be  purchased  in  America,  with 
the  exception  of  antimony,  to  give  the  same  scale  to  within  i°  C. 
Waidner  and  Burgess,  using  both  thermocouples  and  platinum- 


STANDARDIZATION   OF   PYROMETERS  453. 

resistance  pyrometers,  the  latter  of  which  is  capable  of  the  much 
greater  sensitiveness  and  reliability,  have  made  recently  an  ex- 
haustive study  of  the  reproducibility  of  several  of  the  metal 
freezing  points,  as  shown  in  the  following  table,  in  which  the 
samples  were  purchased  from  reliable  American  and  German 
firms  as  their  best  product: 

Metal  Sn          Cd        Pb         Zn         Sb         Al         Cu 

Number  of  samples 5  3         4         3         6          3          4 

Reproducibility*  in  degrees  C.     0.06      .26      .10      .06       2.3       1.2       i.o 

There  was  one  or  more  carefully  analyzed  sample  of  each  metal, 
and  this  table  shows  that,  with  the  exception  of  Sb  and  Al,  it  is 
very  easy  to  get  these  metals  pure  enough  from  several  sources. 
The  only  sufficiently  pure  antimony  was  "  Kahlbaum,"  and  the 
best  aluminium  was  from  the  Aluminium  Company  of  America. 
The  uncertainty  noted  in  the  copper  point  is  due  mainly  to  oxida- 
tion and  the  uncertainties  of  measurement.  Of  the  other  metals 
often  used,  but  not  cited  in  the  above  table,  silver  and  gold  are 
readily  obtained  of  the  highest  purity;  palladium  and  platinum 
less  readily  so,  but  their  purity  as  wires  is  easily  tested  by  meas- 
uring their  temperature  coefficients  (see  Chapter  V). 

Temperature  of  the  Arc  and  Sun.  —  In  certain  problems  in- 
volving extremely  high  temperatures  and  as  comparison  sources 
for  apparent  stellar  temperatures,  the  positive  crater  of  the 
carbon  arc  and  the  sun's  disk  may  be  used,  although  the  actual 
values  to  assign  to  their  temperatures  are  still  somewhat  in 
doubt.  In  the  case  of  measurements  in  terms  of  the  radiation 
laws,  it  is  to  be  remembered  that,  other  things  being  equal  and 
barring  luminescence,  the  values  found  will  be  low,  due  to  the 
selective  radiation  of  carbon  and  of  the  sun's  disk,  or  the  de- 
parture of  their  radiation  from  the  laws  of  the  black  body.  We 
should  expect,  also,  that  measurements  made  by  total-radiation 
methods  would  give  lower  temperatures  than  by  spectral- radia- 
tion methods  if  the  arc  and  sun  have  energy  distributions  differ- 
ing from  those  of  the  black  body  at  the  same  temperatures. 

*  The  reproducibility  is  defined  as  the  average  deviation  of  the  freezing 
points  of  the  metals  from  their  mean  freezing  point. 


454 


HIGH  TEMPERATURES 


The  following  measurements  have  been  made  on  the  positive 
crater  of  the  arc: 


TEMPERATURE  OF  THE  CARBON  ARC. 


Observers. 

Le  Chatelier. . . 
Violle  . . 


Wilson  and  Gray 
Petavel..  . 


Date. 

1892 

1895 


Temper- 
ature 

centigrade. 
4100° 
3600 

3330 
3830 


Wanner 
Very 


Fery 
Fery 


1900    3430-3630 

I 

1899     333°~373° 
348-3930 


3490 


Waidner  and 
Burgess 


1902 
1904 

1904 


(  3343 
3420 


3450 


Method. 

Photometric;  intensity  of  red  light. 

Calorimetric;  specific  heat  of  carbon. 

Total  radiation  of  copper  oxide,  em- 
pirical relation  for. 

Total  light  from  Pt;  empirical  for- 
mula. 

(Varying  with  carbons  used)  photo- 
metric in  terms  of  Wien's  law,  page 

251- 
Wien's  displacement  law. 

Wien's  displacement  law. 

Total  radiation;  Stef  an-Boltzman 

law. 

Photometric;  Wien's  law. 
Total  radiation. 
Holborn-Kurlbaum 

pyrometer  (red 

and  green  light). 
Wanner  pyrometer. 
Le  Chatelier  optical 

pyrometer. 


Wien's  law. 


It  seems  probable  that  the  temperature  of  the  arc  is  not 
over  3600°  C.,  and  the  value  3500°  C.  appears  to  best  represent 
the  results.  Considerable  changes  in  current  produce  less  effect 
on  the  apparent  temperature  than  do  variations  in  the  kind  of 
carbons  used.  In  taking  observation  in  the  arc,  it  is  convenient 
to  mount  the  positive  terminal  horizontally  and  the  negative 
vertically.  Care  should  be  taken  to  have  a  sufficient  area  at 
the  maximum  temperature,  especially  when  using  total-radiation 
methods.  This  can  only  be  accomplished  by  using  heavy  car- 
bons, 1.5  cm.  or  more  in  diameter,  and  correspondingly  high 
currents. 

Observations  in  terms  of  several  of  the  radiation  laws  have 
been  made  on  the  apparent  temperature  of  the  sun's  disk.  Ob- 
servation shows  also  that  the  apparent  temperature  falls  off  from 
the  center  to  the  limb,  due  to  absorption  in  the  outer  layers,  from 
which  it  is  deduced  that  the  photosphere  has  a  temperature  some 
500°  hotter  than  the  observed  value  for  the  center  of  the  disk. 


STANDARDIZATION  OF  PYROMETERS  455 

(A)  If  Stefan's  law  is  assumed  to  hold,  and  if  the  solar  con- 
stant /  as  well  as  the  coefficient  a  are  known,  the  formula  /  =  oT4, 
with  a  proper  choice  of  units,  gives  us  T  (absolute)  directly;  or 
a  calibrated  total-radiation  pyrometer  may  be  used  if  the  absorp- 
tion of  the  earth's  atmosphere  is  corrected  for. 

(B)  If  the  position  of  the  wave  length  of  maximum  energy  is 
known  and  if  the  spectral-energy  curve  for  the  sun  resembles 
that  of  the  black  body,  Wien's  displacement  law  \mT=  C  may 
be  used  if  C  is  known. 

(C)  Similarly,  the  relation  EmT~b  =  const,  may  also  be  used. 

/   c,        \-i 

(D)  Finally,  Planck's  equation,  I  =  Ci\-5(^T  -  i  J   ,  gives  us 
still  another  method  if  c%  is  known. 

If  these  methods,  including  measurements  with  several  wave 
lengths  by  (D),  all  gave  the  same  temperature  for  the  sun, 
using  the  constants  characteristic  of  the  black  body  in  the 
several  equations,  it  would  follow  that  the  apparent  temperature 
found  would  be  the  true  temperature  of  the  sun.  The  spectral 
methods  in  general,  however,  appear  to  give  relatively  high 
values,  indicating  that  the  true  temperature  of  the  sun,  except 
for  luminescent  effects,  is  higher  than  any  of  the  observed 
values. 

Some  of  the  recent  observations  are  given  below  for  the  appar- 
ent mean  value  of  the  temperature  of  the  sun's  disk. 

SOME  RECENT  ESTIMATES  OF  THE  SUN'S  APPARENT 
TEMPERATURE. 

Mean 

Observers.          temperature  Method. 

centigrade. 

Millochau  and     j  (  (A)  With  actinometer  (solar  constant=2.8  to 

Fery  .........  j"  5°9°-539o  }      2  55)  an(j  total-radiation  pyrometer. 

Scheiner  .......          593°         (A) 

Wilsingand         )  5130-5600     (D)  Using  5  wavelengths,  c2=  14,600. 


M      ,  v_^™  J  Modification  of  (D)  with  heterochrome  pho- 

Nordmann  .....     5050-5630  j      tometer,  assumes  arc  =  3343°  C.,  c2=  14,600. 

Abbot  and  Fowle      6160         (B)  Xm=  0.433  M  I  C=  2930. 

(  (A)  Solar  constant  =  i.  95  ;Kurlbaum's  value  of 
Abbot  and  Fowle      55?o      {       >  (page  ^ 

5460  or  j  (D)  Using  several  wave  lengths  and  for  a, 
r  I4j200 


456  HIGH  TEMPERATURES 

Goldhammer  has  shown  that  (B)  is  probably  the  least  reliable 
method,  and  (D)  the  one  least  subject  to  objection  if  several 
wave  lengths  are  used.  For  measurements  corrected  for  the 
earth's  atmosphere,  the  value  6000°  C.  would  seem  to  be  a  fair 
one  for  comparison  with  other  celestial  sources. 

Table  of  Fixed  Points.  —  In  the  actual  state  of  our  knowledge, 
the  fixed  points  to  which  we  should  give  preference  are  summa- 
rized in  the  table  below,  in  which  temperatures  below  1600°  C. 
are  expressed  in  terms  of  the  scale  of  the  nitrogen  constant- 
volume  thermometer,  which  has  given  fairly  consistent  results 
to  1100°  C.,  as  we  have  seen,  in  the  hands  of  several  experimenters. 
Between  1100°  and  1600°  C.  the  results  of  Day  and  Sosman  are 
followed,  and  above  1600°  temperatures  are  expressed  in  terms 
of  Wien's  law  (page  251),  in  which  c2  is  taken  as  14,500,  as  best 
representing  the  data  at  hand.  Estimates  of  the  accuracy  with 
which  these  fixed  points  are  known,  and  also  of  their  reproduci- 
bility  from  a  known  source  of  supply,  are  given  in  the  table.  The 
uncertainty  of  some  measuring  device  is  of  course  included  under 
reproducibility. 

TABLE  OF  FIXED  POINTS. 

Boiling  points.        Accuracy. 


Naphthalene  ......................     218.0  0.2  .02 

Benzophenone  ....................     306  .  o  0.3  .03 

Sulphur  ...........................     444-7  °-  5  •  °3 

Freezing  points. 

Tin  ...............................      231.9  0.2  0.03 

Cadmium  .........................     321  0.3  0.05 

Lead  .............................     327  0.3  0.05 

Zinc  ..............................     419  0.5  0.05 

Antimony  .........................     631  1.5  0.3 

Sodium  chloride  ..................     800  2.0  i  .  o 

Silver  ............................     961  2.0  0.3 

Gold  ............  .  ................   1063  3.0  0.5 

Copper  ...........................   1083  3  i 

Lithium  metasilicate  ..............   1202  5  2 

Diopside  ..........................   1391  10  5 

Nickel  ............................   1450  15  10 

Palladium  ........................   1550  15  5 

Platinum  .........................   1755  20  10 

Tungsten  .........................   3000  100  25 

Carbon  arc  .......................  3500  150  50 

Sun  ...............................   6000  500  100 


STANDARDIZATION  OF   PYROMETERS  457 

Standardization  of  Pyrometers.  —  The  above  discussion  has 
shown  that  we  possess  a  number  of  fixed  points  which  have  been 
established  with  sufficient  accuracy  to  use  them  in  the  stand- 
ardization of  pyrometers.  For  such  standardization,  two  courses 
are  open  besides  direct  comparison  with  a  gas  thermometer,  a 
proceeding  usually  out  of  the  question,  and  furthermore  rendered 
superfluous  by  the  establishment  of  these  fixed  points  in  terms 
of  the  gas  scale.  When  its  construction  permits,  a  pyrometer 
may  be  calibrated  by  finding  its  indications  at  two  or  more  of 
the  fixed  points,  or  may  be  compared  with  another  which  has 
been  so  calibrated.  The  latter  method  is  the  one  used  for  ordi- 
nary purposes,  as  in  the  graduation  of  industrial  instruments,  but 
for  pyrometers  which  are  to  be  used  as  standards  the  former 
method  should  be  used  when  possible. 

We  have  discussed  at  some  length,  in  their  respective  chap- 
ters, the  methods  of  calibration  for  the  various  pyrometers,  and 
it  is  unnecessary  to  dwell  further  on  this  matter,  except  to  say 
that  it  cannot  be  assumed  that  a  pyrometer  once  standardized 
is  standardized  for  all  time,  especially  if  it  is  subjected  to  hard 
usage. 

Standardizing  Laboratories.  —  Recognizing  the  importance  of 
establishing,  preserving,  and  disseminating  a  common  and  author- 
itative temperature  scale  and  of  providing  means  of  having 
pyrometers  and  other  instruments  certified  as  to  their  accuracy, 
some  of  the  governments  have  established  laboratories,  such 
as  the  Physikalisch-Technische  Reichsanstalt  in  Germany,  the 
National  Physical  Laboratory  in  England,  the  National  Bureau  of 
Standards  in  the  United  States,  and  the  Laboratoire  d'Essais  and 
the  Laboratoire  Central  d'Electricite  in  France,  whose  functions 
are  not  only  testing  instruments  but  carrying  on  researches  as 
well.  The  German  institution,  the  oldest  of  these  laboratories, 
has  been  one  of  the  most  potent  factors  in  the  development  of 
excellency  in  German  instruments,  and  has  been  of  immense 
service  to  the  industries  as  well  as  to  the  interests  of  science;  and 
the  other  national  laboratories  are  fast  assuming  a  position  of 
equal  importance  in  their  respective  countries. 


458  HIGH  TEMPERATURES 

Metals  and  Salts  of  Certified  Melting  Points.  —  It  would  often 
be  of  great  convenience,  when  one  has  to  calibrate  his  own  pyrom- 
eter, and  in  cases  of  dispute  between  individuals  as  to  their 
respective  temperature  scales,  to  have  available  metals  or  salts 
the  melting  points  of  which  had  been  certified  by  a  standardizing 
laboratory.  The  Bureau  of  Standards  is  preparing  to  issue  such 
certified  metals  and  salts  of  sufficient  range  and  number  to  meet 
the  ordinary  requirements  of  pyrometer  calibration. 

Electrically  Heated  Furnaces.  —  For  the  standardization  of 
pyrometers  as  well  as  in  many  other  high-temperature  problems, 
it  is  necessary  to  preserve  a  constant  temperature  for  a  consider- 
able time  and  to  be  able  to  reproduce  a  given  temperature  very 
exactly. 

Electrically  heated  resistance  furnaces  best  serve  these  ends, 
and  great  improvements  have  been  made  in  their  construction 
in  recent  years. 

Furnaces  wound  with  nickel  wire  of  i  to  2  mm.  diameter  on 
porcelain  have  been  used  considerably,  but  they  are  slow  in  heat- 
ing up  and  their  upper  limit  is  about  1200°  C.,  if  the  furnace  is 
to  be  used  frequently,  although  for  a  single  heating  1400°  C.  may 
be  attained  with  care.  Platinum  wire  has  been  used  to  attain 
.higher  temperatures,  but  the  use  of  this  material  in  wire  form  is 
very  expensive  for  heating. 

Heraeus  has  made  electric  heating  to  1300°  or  1450°  C.,  depend- 
ing on  size  of  furnace,  generally  accessible  by  the  substitution  of 
platinum  foil  for  the  wire,  weighing  about  1.5  grams  per  square 
centimeter  or  having  a  thickness  of  about  0.007  mm.  This  re- 
duces the  cost  of  a  platinum  furnace  very  greatly,  and  has  the 
further  advantages  of  giving  slightly  greater  uniformity  of  heat- 
ing and  attaining  at  a  somewhat  greater  speed  high  temperatures 
than  with  wire- wound  furnaces.  Above  1500°  C.  chemical  action 
sets  in  between  the  platinum  and  material  of  the  tubes  usually 
.employed,  so  that  although,  as  far  as  the  platinum  is  concerned, 
1700°  C.  could  be  maintained  for  a  short  time,  yet  the  present 
safe  upper  limit  for  long  periods  of  heating  is  1400°  with  foil 
furnaces. 


STANDARDIZATION  OF   PYROMETERS 


459 


Their  greatest  weakness  is  the  cracking  of  the  porcelain  tubes 
on  which  the  foil  is  wound  and  the  evaporation  of  the  platinum 
when  not  covered  with  a  suitable  paste. 

For  very  high  temperatures,  up  to  2100°,  the  iridium-tube 
furnaces  of  Heraeus  may  be  used,  as  they  have  been  with  success 
by  Nernst  and  others  in  the  study  of  vapor  pressures  at  these 
temperatures,  as  well  as  in  melting  point  and  physiochemical 
investigations.  In  Fig.  174  is  shown  the  furnace  and  accessories 
used  by  Waidner  and  Burgess  for  the  determination  of  the  palla- 


Fig.  174.     Iridium-tube  Furnace. 


dium  and  platinum  melting  points.  For  lower  temperatures, 
tubes  of  platinum  or  of  a  platinum  alloy  may  be  used  with  great 
saving  in  cost. 

Crucible  Furnaces.  —  A  suitable  form  of  electrically  heated 
crucible  furnace  for  freezing  and  melting  point  determinations  to 
1100°  C.,  such  as  used  at  the  Bureau  of  Standards,  is  shown  in 
Fig.  175.  The  double  winding  with  platinum  ribbon  gives  a  very 
delicate  temperature  control  if  connected  in  parallel  through  sep- 
arate rheostats  on  the  same  battery.  This  furnace  is  designed 
to  carry  a  crucible  of  300  c.c.  capacity,  so  as  to  give  ample  im- 


460 


HIGH  TEMPERATURES 


mersion  of  the  thermometer  at  a  constant  temperature.  If  the 
freezing  or  melting  of  a  metal  is  made  to  take  twenty  minutes  or 
more,  the  form  of  the  freezing  or  melting  curve  becomes  a  very 


10  cm 
Fig.  175.     Double- wound  Crucible  Furnace. 

sensitive  check  on  the  purity  of  the  sample.  Another  form  of 
crucible  furnace  used  successfully  at  the  Carnegie  Geophysical 
Laboratory  to  above  1600°  C.  is  shown  in  Fig.  52.  The  char- 
acteristic of  this  furnace  is  the  inwound  platinum-wire  heating 


STANDARDIZATION  OF  PYROMETERS 


461 


coil.  Although  of  high  first  cost,  it  is  a  very  durable  furnace  as 
constructed.  Both  types  of  furnace  may  be  arranged  for  use 
with  any  desired  atmosphere.  Less  satisfactory  results  will  be 
obtained  with  ordinary  gas  furnaces  to  1300°  C. 


SECTION  A-B 


Fig.  176.     Arsem  Vacuum  Electric  Furnace. 

Vacuum  and  Pressure  Furnaces.  —  Mr.  Arsem  of  the  General 
Electric  Company  has  developed  a  type  of  vacuum  furnace  which 
is  convenient  for  certain  melting  points,  such  as  fireclays,  re- 
fractory bricks,  and  ashes,  and  chemical  investigations  to  2500°  C. 
or  higher.  In  one  of  its  ordinary  water-cooled  forms,  shown  in 
Fig.  176,  the  heating  is  produced  by  passing  an  alternating  cur- 


462 


HIGH   TEMPERATURES 


rent  of  low  voltage  through  a  graphite  spiral.  The  highest  tem- 
peratures may  be  reached  in  a  few  minutes.  The  interior  is 
observed  through  a  mica  or  glass  window.  Temperatures  are 
measured  with  an  optical  pyrometer.  By  taking  the  heating 
curve,  the  transformation  points  for  only  a  few  tenths  grams  of 
material  are  easily  observed.  (See  page  342.)  Such  a  furnace 
has  been  in  constant  use  at  the  Bureau  of  Standards  for  several 
years. 

V.  Wartenberg  has  successfully  constructed  a  tube  resistance 
furnace  of  tungsten  mounted  in  vacuo  (Fig.   177),  and  with  it 


Pump 


Fig.  177.     Wartenberg's  Tungsten  Furnace. 


determined  the  melting  points  of  a  number  of  refractory  elements 
melting  above  2000°  C. 

Messrs.  Hutton  and  Petavel  of  Manchester,  England,  have 
constructed  a  pressure  furnace  for  work  at  high  temperatures, 
the  essential  parts  of  which  are  shown  in  Fig.  178.  A  vertical 
carbon  tube  was  electro-coppered  at  the  ends,  soldered  into 
brass  castings,  and  provided  with  water  circulation  at  A  and  B. 
Temperature  readings  were  taken  down  the  side  tube  of  carbon, 
fixed  into  a  brass  tube  with  a  window  at  the  end,  a  current  of 
hydrogen  being  admitted  at  C.  The  whole  furnace  was  packed 
in  crushed  wood  charcoal,  while  a  thin  walled  graphite  crucible 


STANDARDIZATION  OF  PYROMETERS 


463 


contained  the  metal  to  be  studied.     This  furnace  has  been  used 
by  Greenwood  for  the  determination  of  the  boiling  points  of 


Fig.  178.     Graphite  Furnace  of  Hutton  and  Petavel. 

some  of  the  metals  and  their  variation  with  pressure.     Other 
types  of  furnace  are  described  in  Chapters  II,  IV,  and  V. 


BIBLIOGRAPHY. 


GENERAL  WORKS. 

(The  heavy  figures  refer  to  volumes.) 
Weinhold.  —  Principles  of  construction  of  pyrometers.     Pogg.  Ann.,  149,  p.  186; 

1873- 
C.  H.  Boh.  —  Die  Pyrometer,  Eine  Kritik  der  bisher  construirten  hoher  Temper- 

aturmesser  in  wissenschaftlich-technische  Hinsicht,  1888. 
C.  Barus.  —  Die  physikalische  Behandlung  und  die  Messung  hoher  Temperaturen, 

1892,  Leipzig.  —  Bull.  U.  S.  Geological  Survey  No.  64,  1889;  No.  103,  1893. 
Kayser.  —  Handbuch  der  Spectroscopie,   Bd.   2,    1902.     Summary  of  radiation 

methods. 
Callendar.  —  Measurement  of  Extreme  Temperatures.     Nature,  69,  1899. 

C.  W.  Waidner.  —  Methods  of  Pyrometry.     Proc.  Eng.  Soc.  Western  Pennsylvania, 

Sept.,  1904. 
Waidner  and  Burgess.  —  Establishment  of  High-temperature  Scale.     Phys.  Rev., 

24,  p.  441;   1907. 
Burgess.  —  Estimation  of  High  Temperatures.      Proc.  Am.  Electroch.  Soc.,  11, 

p.  247;   1907. 

Chivolson.  —  Traite"  de  Physique.     (Instruments,  Theory,  Bibliography.) 
Leber.  — Ch.  xiv  in  Geiger's  Handbuch  der  Eisen-  und  Stahlgiesserei,  1911. 

NORMAL  SCALE  OF  TEMPERATURES. 

Carnot.  —  Reflections  on  the  motive  power  of  fire. 

Lippmann.  — Thermodynamics,  p.  51. 

Thomson  and  Joule.  —  Philosophical  Transactions  of  the  Royal  Society,  42,  p.  579; 

1862. 

Thomson  (Lord  Kelvin).  —  Collected  Papers,  1,  p.  174. 
Lehrfeldt.  —  Philosophical  Magazine,  46,  p.  363;   1898. 
Callendar.  —  Phil.  Trans.,  178,  pp.  161-220;  1888. 
Regnault.  —  Account  of  his  investigations,  1,  p.  168;  1847. 
Chappuis.  —  Studies  of  the  gas  thermometer.     Trav.  du  Bureau  International 

des  Poids  et  Mesures,  6,  1888. 
Chappuis  and  Barker.  —  Trav.  et  M6m.  du  Bureau  Int.  des  Poids  et  Mesures, 

1900.     Phil.  Trans.,  1900. 
Schreber.  —  Absolute  Temperature.     W.  Beibl.,  22,  p.  297;  1898. 

D.  Berthelot.  —  Reduction  of  gas-thermometer  readings  to  the  absolute  scale. 

Trav.  et  Mem.  du  Bureau  Int.,  13,  1903.     C.  R.,  138,  p.  1153;  1904. 
Boltzmann.  —  On  the  determination  of  absolute  temperature.    Wied.  Ann.,  63, 
p.  948;  1894, 

465 


466  HIGH  TEMPERATURES 

E.  Mack.  —  Theory  of  Heat. 

Callendar.  —  Thermodynamical    correction    to   gas   thermometer.      Phil.    Mag., 

May,  1903. 

Rose-Innes.  —  Phil.  Mag.  (6),  2,  p.  130;  1901. 
Pellat.  —  C.  R.,  136,  p.  809;  1903. 
Chappuis.  —  ]\.  de  Phys.,  3,  p.  833;  1904.     Ann.  des  Poids  et  Mesures,  13,  p.  a3; 

1907. 

Berthelot.  — Ann.  des  Poids  et  Mesures,  13,  p.  b3;  1907. 
Buckingham.  —  Bull.  Bureau  Standards,  3,  p.  237;  1907. 
Rose-Innes.  —  Phil.  Mag.,  14,  p.  301;  1908. 


GAS  PYROMETER. 

Prinsep.  —  Ann.  Chim.  et  Phys.,  2d  Series,  41,  p.  247;  1829. 

Poidllet. — Treatise  on  Physics,  9th  ed.,  1,  p.  233;  1858.     Comptes  Rendus,  3, 

p.  782;  1836. 

Ed.  Becquerel.  —  C.  R.,  57,  pp.  855,  902,  955;   1863. 
Sainte-Claire-Devitte  and  Troost.  —  C.  R.,  90,  pp.  727,  773;   1880.     45,  p.  821; 

1857.    49,  p.  239;  1859.     56,  p.  977;  57,  pp.  894,  935;  1863.    98,  p.  1427; 

1884.     69,  p.  162;  1864.    Ann.  Chim.  et  Phys.  (3),  58,  p.  257;  1860.    Rupert. 

Chim.  Appl.,  p.  326;  1863. 
Violle.  —  Specific  heat  of  platinum.     C.  R.,  85,  p.  543;   1877.     Specific  heat  of 

palladium.     C.  R.,  87,  p.  98;  1878.    89,  p.  702;  1879.  —  Boiling  point  of  zinc. 

C.  R.,  94,  p.  721;  1882. 

V.  and  C.  Meyer.  —  Density  of  halogens.     Ber.  D.  Ch.  Ges.,  12,  p.  1426;  1879. 
Earns.— Bull.  U.  S.  Geological  Survey  No.  54, 1889.    Phil.  Mag.  (5),  34,  p.  i;  1892. 

—  Report  on  Pyrometry,  Congress  at  Paris,  1900. 
Regnault. — Account  of  his  experiments,  1,  p.  168;  Paris,  1847.    Mem.  de  ITnstitut, 

21,  pp.  91,  no;  1847.     Ann.  Chim.  et  Phys.  (3),  68,  p.  89;  1861. 
Holborn  and  Wien.  —  Bull,  de  la  Soc.  pour  1'encouragement  (5),  1,  p.  1012;  1896. 

Wied.  Ann.,  47,  p.  107;  1892.    56,  p.  360;  1895.    Zeits.  fur  Instrum,  p.  257;. 

1892. 

Crafts  and  Meier.  —  Vapor  density  of  iodine.  C.  R.,  90,  p.  690;  1880. 
Langer  and  V.  Meyer.  —  Pyrochemical  Researches  (Brunswick),  1885. 
Joly.  —  Pogg.  Ann.,  Jubelband,  p.  97;  1874. 

Randall.  —  Permeability  of  platinum.     Bull.  Soc.  Chim.,  21,  p.  682;  1898. 
Mallard  and  Le  Chatelier.  —  Ann.  des  Mines,  4,  p.  276;  1884. 
/.  R.  Erskine  Murray.  —  On  a  new  form  of  constant-volume  air  thermometer. 

Edinburgh  Proc.,  21,  p.  299;  1896-97.    Journ.  Phys.  Chem.,  1,  p.  714;  1897. 
/.   Rose-Innes.  —  The  thermodynamic  correction  for  an  air  thermometer,  etc. 

Nature,  58,  p.  77;  1898.    Phil.  Mag.  (5),  45,  p.  227;  1898.    60,  p.  251;  1900. 

Proc.  Phys.  Soc.  London  (i),  16,  p.  26;  1898. 
Chappuis.  —  Phil.  Mag.  (5),  50,  p.  433;  1900.     (6),  3,  p.  243;  1902.     Report  for 

Paris  Congress,  1900;  Jour,  de  Phys.,  Jan.,  1901,  p.  20. 
D.  Berthelot. — On  a  new  method  of  temperature  measurement.     C.  R.,  120, 

p.  831;  1895,    Ann.  Chim.  et  Phys.  (7),  26,  p.  58;  1902. 


BIBLIOGRAPHY  467 

Holborn  and  Day. — Wied.  Ann.,  68,  p.  817;  1899.     Am.  Jour.  Sci.  (4),  8,  p.  165; 

1899.     Zeitscher.  Instrum.,  May,  1900.     Am.  Jour.  Sci.  (4),  10,  p.  171;  1900. 

Drude's  Ann.,  2,  p.  505;  1900. 
Chappuis  and  Harker.  —  Trav.  et  Mem.  du  Bureau  Int.  des  Poids,  1900,  1902.  — • 

Phil.  Trans.,  1900. 

Callendar.  —  Phil.  Mag.,  48,  p.  519;  1899.     Proc-  Rov-  Soc.,  50,  p.  247;  1891. 
Callendar  and  Griffiths.  —  Phil.  Trans.,  182,  1891. 
D.  Berthelot.  —  On  gas  thermometers- and  the  reduction  of  their  indications  to  the 

absolute  scale.     Trav.  et  Mem.  du  Bureau  Int.,  13,  1903. 
Travers.  —  Studies  in  Gases  (Macmillan).  —  Proc.  Roy.  Soc.,  70,  p.  485. 
Kapp.  — Ann.  der  Phys.,  5,  p.  905;  1901. 
Wiebe  and  Bb'ttcher.  —  Gas  thermometry,  Berich.     Berlin  Akad.,  44,   p.   1025. 

Inst'kunde,  1888. 

Jaquerod  and  Perrot.  — Various  gases  in  quartz.     C.  R.,  138,  p.  1032;  1904. 
Day  and  Clement.  —  Am.  Jl.  Sci.,  26,  p.  405;  1908. 
Day  and  Sosman.  —  Scale  400°  to  1550°  C.     Am.  Jour.  Sci.  (4),  29,  p.  93;   1910. 

Pub.  157,  Carnegie  Institution  of  Washington,  1911. 
A.  L.  Day.  —  Met.  and  Chem.  Eng.,  8,  p.  257;  1910. 
Holborn  and  Valentiner.  — Ann.  der  Phys.  (4),  22,  p.  i;  1907. 
Berthelot.  —  Value  of  gas  constant.     Zs.  Elektroch.,  10,  p.  621;   1904. 
Jaquerod  and  Perrot.  —  Expansion  coefficients.     C.  R.,  140,  p.  1542;  1905. 
Holborn  and  Henning.  —  Ann.  der  Phys.  (4),  35,  p.  761;  1911. 

CALORIMETRIC   PYROMETER. 

(See  also  Specific  Heat.) 

Violle.  —  Boiling  and  fusing  points.     C.  R.,  89,  p.  702;  1879. 

Le  Chatelier.  —  Sixteenth  Congress  of  the  Societe"  technique  de  1'industrie  du  gaz, 

June,  1889. 

Euchtne.  —  Thermal  relations  in  the  distillation  of  oil.     (Monograph.) 
Ferrini.  —  Rend.  Lomb.,  35,  p.  703;  1902. 
Berthelot.  —  Calorimetry.     Ann.  Chim.  et  Phys.  (4),  20,  p.  109;  (5),  5,  p.  5;  (5), 

10,  pp.  433,  447;  (5),  12,  p.  550. 
Hoadley.  —  Trans.  Am.  Inst.  Mech.  Engs.,  2,  No.  23,  p.  42;  3,  No.  65,  p.  187. 

THERMOELECTRIC   PYROMETER. 

Becquerel.  —  Ann.  Chim.  et  Phys.  (2),  31,  p.  371;  1826. 

Pouillet.—Traite  de  Physique,  4th  ed.,  2,  p.  684;  C.  R.,  3,  p.  786;  4,  p.  513;  1836. 

Ed.  Becquerel.  —  Annales  du  Conservatoire,  4,  p.  597;  1864.     C.  R.,  65,  p.  826; 

1862.     Ann.  de  Chim.  et  de  Phys.  (3),  68,  p.  495  1863. 

Tail.  —  Thermoelectric  power.     Trans.  Roy.  Soc.  Edinb.,  27,  p.  125;  1872-73. 
Regnault.  —  Account  of  investigations  on  heat  engines,  1,  p.   240.     C.  R.,  21, 

p.  240;  1847.     (Fe-Pt  couple.) 
Knott  and  MacGregor.  —  Graphical  representations.     Trans.  Roy.  Soc.  Edinb.,  28, 

p.  321;  1876-77. 
Seebeck.  —  Fundamental  principles.     Pogg.  Ann.,  6,  pp.  133,  263;  1826. 


468  HIGH  TEMPERATURES 

Gumming.  —  Inversion  points.     Trans.  Cambridge  Soc,  2,  p.  47;  1823. 

Draper.  —  Phil.  Mag.  (3),  16,  p.  451;  1840. 

Mousson.  —  Arch.  d'Elec.  (Geneva),  4,  p.  5;  1844.     (Inhomogeneity.) 

Magnus.  —  Pogg.  Ann.,  83,  p.  469;  1851.     (Inhomogeneity.) 

Thomson.  —  Trans.  Edinb.  Soc.,  21,  p.  123;    1854.     Phil.  Mag.  (4),  11,  pp.  214, 

281,  379,  433;  1856.    C.  R.,  39,  p.  116;  1854.    Phil.  Trans.,  146,  p.  649;  1856. 
Avernarius.  —  Formulae  and  various  couples.     Pogg.   Ann.,  119,   p.   406;   1863. 

122,  p.  193;  1864. 

Kleminic  and  Czermak.  —  Pt-Cu-Ni-Fe  couples.     Wied.  Ann.,  50,  p.  175;   1893. 
Le  Chatelier.  —  Thermoelectric  pyrometer.     C.  R.,  102,  p.  819;   1886.     Journal 

de  Phys.  (2),  6,  Jan.,  1887;  Genie  civil,  March  5,  1887;  i6th  Congress  of  the 

Societe  technique  de  1'industrie  du  gaz,  June,  1889;  Bull,  de  la  Societe  de 

I'encouragement,  1892;  Bull.  Soc.  Chim.  Paris,  47,  p.  42;  1887. 
Barus.  —  Washington,  1889,  Bull,  of  the  U.  S.  Geological  Survey,  No.  54,  and 

No.  103  (No.  54  contains  a  very  complete  historical  account  of  the  whole 

subject  of  pyrometry).    Phil.  Mag.  (5),  34,  pp.  15,  376;  1892.     Am.  Jour.  Sci., 

36,  p.  427,  1888;  47(3),  p.  366;  48,  p.  336;  1894.    (Pt-  Pt-Ir  couples;  formulae.) 
Holbornand  Wien.  — Wied.  Ann.,  47,  p.  107;  1892.     66,  p.  360;  1895.     Zeit.  des 

Vereines  deutscher  Ingenieure,  41,  p.  226;  1896.     Stahl  und  Eisen,  16,  p.  840. 

(Pt  •  Pt-Rh  couples.) 
Roberts-Austen.  —  Recent  progress  in  pyrometry.     Trans.  Am.  Institute  of  Mining 

Engineers,  1893.     (See  also  Recording  Pyrometers.) 
E.  Damour.  —  Bull,  de  1'Assoc.  amicale  des  anciens  eleves  de  1'Ecole  des  Mines, 

March,  1889. 
H.  Howe.  —  Pyrometric  data.     Engineering  and  Mining  Journal,  50,  p.  426;  1890. 

Metallurgical  laboratory  notes  (expts.  with  thermocouples),  1902. 
Holborn  and  Day. — Pt   alloys.     Ann.   d.  Phys.    (4),  2,  p.   505;    1900.     On   the 

melting  point  of  gold.    Ann.  d.   Phys.  (4),  p.  99;   1901.     Am.  Jour.   Sci., 

11,   p.    145;    1901.  —  On  the   thermoelectric  properties  of  certain  metals. 

Sitz.  Berl.  Akad.,  p.  69;  1899.     Am.  Jour.  Sci.  (4),  (8),  p.  303;  1899.    Mitth. 

Phys.-tech.  Reichsanst,  37,  1899. 

Stansfield.  —  Pt  alloys,  formulae.     Phil.  Mag.  (5),  46,  p.  59;  1898. 
Holman.  —  Exponential  formulae.     Phil.  Mag.,  41,  p.  465;  1896.     Proc.  Am.  Acad., 

31,  p.  234. 
Holman,  Lawrence,  and  Ban.  —  Phil.  Mag.,  42,  p.  37;  1896.     Proc.  Am.  Acad.,  31, 

p.  218. 

Schoentjes.  —  Arch,  de  Phys.  (4),  5,  p.  136;  1898. 

Noll.  —  Thermoelectricity  of  chemically  pure  metals.     Wied.  Ann.,  p.  874;  1894. 
Steinmann. — Thermoelectricity  of  certain  alloys.     C.   R.,  130,   p.    1300;   131, 

p.  34;  1900. 
Belloc.  —  Thermoelectricity  of  steels.     C.  R.,  131,  p.  336;  1900.     Ann.  Chim.  et 

Phys.,  30,  p.  42;  1903.     C.  R.,  134,  p.  105;  1902. 

D.  Berthelot.  —  On  the  graduation  of  couples.     C.  R.,  134,  p.  983;  1902. 
Thiede.  —  Freezing-point  apparatus.     Zs.  Angew.  Ch.,  16,  p.  780;  1902. 
Lindeck  and  Rothe.  —  The  standardization  of  thermoelements  for  high-temper- 
ature measurements  (as  carried  out  at  the  Phys.  Tech.  Reichsanstalt).     Zs. 

Instrument'k.,  20,  p.  285;  1900,  and  following  years. 


BIBLIOGRAPHY  469 

Nichols.  —  Temperature  of  flames,  etc.,  with  thermocouple.    Phys.  Rev.,   10, 

p.  234;  1900. 

Harker.  —  Gas  vs.  thermoelectric  scales.     Phil.  Trans.,  203,  p.  343;  1904. 
Chauvin  and  Arnoux.  — Compound  couple.     Bull.  Soc.  d'Encourag.,  109,  p.  1171; 

1907. 

Crompton.  —  Fe-Ni,  Fe-Cu.     Lond.  Electrician,  66,  p.  808;  1906. 
Bristol.  —  Pyrometers.     Am.  Soc.  Mech.  Eng.,  27,  p.  552;  1906.     Am.  Machinist, 

p.  201;  1906.     Electrochem.  and  Met.  Ind.,  4,  p.  115;  1906. 
Pecheux.  —  Le  pyrome'tre  thermoele"ctrique,  Paris,  1909.     (Ni-Cu,  etc.,  couples.) 

C.  R.,  139,  p.  1202;  1904.     148,  p.  1041;  1909.     149,  p.  1062;  1909. 
Broniewski.  —  Thermoelectric  properties  of  alloys  (with  a  bibliography,  1822-1909). 

Rev.  de  Metallurgie,  7,  p.  45;  1910. 

C.  H.  Wilson.  —  Electrochem.  and  Met.  Ind.,  7,  p.  116;  1909. 
Barrett.  —  Phil.  Mag.  (5),  49,  p.  309;  1900.     (Fe-Ni-Mn  to  1000°  C.) 
Harrison.  —  Fe-Ni-Cu  alloys  to  1050°  C.     Phil.  Mag.  (6),  3,  p.  177;  1902. 
Boudouard.  —  Steels  o-i2oo°C.     Rev.  d.  M6tallurgie,  1,  p.  80;  1904. 
Hevesy  and  Wolf.  —  Ni-Ag  couple.     Phys.  Zs.,  11,  p.  473;  1910. 
Stupakoff.  —  Pamphlets  on  Industrial  Practice. 
Schneider.  —  Thermoelectric  powers  of  heated  wires.     Elektrotech.  Zs.,  25,  p.  233; 

1904. 

Hirschson.  —  C-Ni  couple.     Zs.  Chem.  Apparatk.,  p.  622;  1907. 
Sosman.  —  Pt-Rh.     Am.  Jour.  Sci.,  30,  p.  i;  1910. 
Day  and  Allen.  —  Pt-Rh  to  1600°  C.     Phys.  Rev.,  19,  p.  177;  1904. 
Palmer.  —  Fe-Cu  0-200  .     Phys.  Rev.,  21,  p.  65;  1905. 
A.  Campbell.  —  Composite  thermocouples.     Phil.  Mag.,  9,  p.  713;  1905. 
White.  —  Constancy  of  thermoelements.     Phys.  Rev.,  23,  p.  449;  1906.     Melting- 
point  methods.     Am.  Jour.  Sci.,  28,  p.  453;  1909. 

Geibel. — The  noble  metals.     Zs.  Anorg.  Ch.,  69,  p.  38;  1910.     70,  p.  240;  1911. 
Waidner  and  Burgess.  —  Comparison   of   thermoelectric   and  resistance   scales. 

Bull.  Bur.  Standards,  6,  p.  182;  1910. 
Memmler  and  Schob.  —  Mitt.  K.  Material  priifungsamt,  28,  p.  307;  1910.     (Use  in 

testing  machine.) 

CORRECTIONS  FOR  COLD  JUNCTIONS. 

C.  Ojferhaus  and  E.  H.  Fischer.  —  Electrochem.  and  Met.  Ind.,  6,  p.  362;  1908. 

R.  Vogel.  —  Zs.  Anorg.  Chem.,  46,  p.  13;  1905. 

Schultze  and  Koepsel.  —  Centralb.  Accumulatoren,  8,  p.  102;  1907. 

POTENTIOMETERS  FOR  TEMPERATURE  MEASUREMENTS. 

Fuessner.  — Zs.  f.  Instrumentenkunde,  10,  p.  113;  1890. 

Lehrfeldt.—  Phil  Mag.,  6,  p.  668;  1903. 

Varley.  —  Brit.  Assoc.,  36,  p.  14;  1866. 

Raps.  —  Zs.  f.  Instrumentenkunde,  16,  p.  215;  1895.     Elektrotech.  Zs.,  16,  p.  507; 

1895.     24,  p.  978;  1903. 

Franke.  —  Elektrotech.   Zs.,  24,  p. 978.   Zs.  f.  Instrumentenkunde,  24,  p.  93;  1904. 
Harker.  —  Phil.  Mag.,  6,  p.  41;  1903. 


470  HIGH  TEMPERATURES 

Holman.  —  Phil.  Mag.,  42,  p.  37;   1896. 

Lindeck. — Zs.  f.  Instrumentenkunde,  20,  p.  293;  1900. 

Hausrath.  —  Ann.  der  Phys.,  17,   p.  735;  1905.     Zs.  f.  Instrumentenkunde,  27, 

p.  309;  1907. 

Diesselhorst.  — Zs.  f.  Instrumentenkunde,  26,  pp.  173,  297;  1906.  28,  p.  i;  1908. 
White.  —  Zs.  f.  Instrumentenkunde,  27,  p.  210;  1907.  Phys.  Rev.,  25,  p.  334;  1907. 
Wenner.  —  Phys.  Rev.,  31,  p.  94;  1910. 


ELECTRICAL  RESISTANCE   PYROMETER. 

Muller.  —  Pogg.  Ann.,  103,  p.  176;  1858. 

W.  Siemens.  —  Proc.  Royal  Soc.,  19,  p.  351;  1871.  Bakerian  Lecture,  1871.  Trans- 
actions of  the  Society  of  Telegraph  Engineers,  1879.  British  Association, 
p.  242;  1874. 

Benoit.  —  C.  R.,  76,  p.  342;  1873. 

Callendar.—PhiL  Trans,  of  R.  S.,  178,  pp.  160-230;  1888.  Proc.  Roy.  Soc.  Lond., 
41,  p.  231;  1886.  Phil.  Trans.,  1887.  Phil.  Trans.,  p.  119;  1892  (with 
Griffiths').  —  Platinum  pyrometers.  Iron  and  Steel  Institute,  May,  1892. 
Phil.  Mag.,  32,  p.  104;  1891.  33,  p.  220;  1892.  —  Proposals  for  a  standard  scale 
of  temperatures.  B.  A.  Report,  1899;  Phil.  Mag.,  47,  pp.  191,  519;  1899,  is 
a  r6sume  of  the  question.  Phil.  Trans.,  199,  p.  i ;  1902.  —  Steam  temperatures. 
Br.  Assoc.  Rept.,  p.  422;  1897.  (Ibid,  with  Nicholson)  Proc.  Inst.  C.  E.,  p.  131; 
1898.  —  Gas-engine  Cylinders,  with  Dalby.  Engineering  (Lond.),  84,  p.  887; 
1907. 

Heycock  and  Neville.  —  Determination  of  high  temperatures.  Jour,  of  Chem. 
Society,  67,  pp.  160,  1024;  1895.  Phil.  Trans.,  189, p.  25;  1897.  202,  pp.  1-69. 

Earns.  —  Amer.  Jour.  Sci.  (3),  36,  p.  427;  1888. 

Holborn  and  Wien. — Ann.  der  Phys.,  47,  p.  107;  1892.  56,  p.  360;  1895.  Bull,  de 
la  Soc.  d'encouragement,  5th  Series,  1,  p.  1012;  1896. 

Chappuis  and  Harker.  —  A  comparison  of  platinum  and  gas  thermometers  made 
at  the  B.  Int.  des  Poids  et  Mesures.  B.  A.  Report,  1899;  Trav.  et  Mem.  du 
Bureau  Int.  des  Poids  et  Mesures,  1900,  1902;  Phil.  Trans.,  1900;  Jour,  de 
Phys.,  10,  p.  20;  1901.  Proc.  Roy.  Soc.,  65,  p.  377;  1899. 

Appleyard.  —  Phil.  Mag.  (5),  41,  p.  62;  1896. 

Dickson.  — Formulae  Phil.  Mag.  (5),  44,  p.  445;  1897.     45,  p.  525;  1898. 

Wade.  —  Wied.  Beibl.,  23,  p.  963;  1899.     Proc.  Cambr.  Soc.,  9,  p.  526;  1898. 

Waidner  and  Mallory.  —  Phys.  Rev.,  8,  p.  193;  1899. 

Barnes  and  Mclntosh.  —  New  form  of  platinum  thermometer.  Phil.  Mag.,  6, 
P-  3535  1903- 

Tory.  —  Br.  Assoc.  Rpt,  p.  588;  1897.     Phil.  Mag.,  50,  p.  421;  1900. 

Whipple.  —  Temperature  indicator,  etc.    Phys.  Soc.  (Lond.),  18,  p.  235;  1902. 

Chree.  —  Platinum  Thermometry  at  the  Kew  Observatory.  Proc.  Roy.  Soc., 
67,  p.  3. 

Harker.  —  On  the  high-temperature  standards  of  the  National  Physical  Labora- 
tory. Proc.  Roy.  Soc.,  73,  p.  217;  1904. 

Jaeger.  —  Precision  of  Resistance  Methods.     Zs.  Instr'kunde,  26,  p.  69;  1906. 


BIBLIOGRAPHY  471 

Burstall.  —  Rapidly  varying  temperatures.     Phil.  Mag.,  40,  p.  282;  1895.     Prbc. 

Inst  Mech.  Engrs.,  p.  1031;  1901. 
Hopkinson.  —  Gas-engine  Temperatures.     Engineering,  81,  p.    777;  1906.     Phil. 

Mag.,  13,  p.  84;  1907. 
H.  Edwards.  —  Contr.   Jefferson  Lab.,  2,   p.  549;   1904.     Proc.  Am.  Acad.,  40, 

No.  14. 
Marvin. — Nickel  to  300°  C.     Phys.  Rev.,  30,   p.    522;   1910.  —  Direct-reading 

Bridge  for  Ni  and  Pt.     Jour.  Frankl.  Inst.,  171,  p.  439;  1911. 
Travers  and  Gwyer.  —  Scale  444°  to  —  190°.     Proc.  Roy.  Soc.,  74,  p.  528;  1905. 

A.  Campbell.  —  Direct  Reading.     Phil.  Mag.,  9,  p.  713;  1905. 

Waidner  and  Burgess. — Pt  and  Pd  to  1 100°  C.    Bull.  Bureau  Standards,  6,  pp.  149- 

230;  1909.     (Contains  bibliography  of  about  135  titles.) 
Holborn  and  Henning.  —  Pt  w.  Gas  Therm,  to  S.B.P.     Ann.  der  Phys.  (4),  26, 

p.  835;  1908.     36,  p.  761;  1911. 

Clark,  Fisher,  and  Wadsworth.  —  Pyrometer  Bridge.    Electrician,  60,  p.  376;  1906. 
Harris.  —  Deflectional  Bridge  Pyrometer.     Electrician,  62,  p.  430;  1908. 
Cambridge  Co.  —  Callendar  and  Griffiths  Bridge.     Electrician,  60,  p.  477;  1906. 
Bruger. — Hartmann  and  Braum  Instrument.     Elektrotech.  Zs.,  27,  p.  531;  1906. 

E.  F.   Northrup.  —  Measurement  of   Temperature  by  Electrical  Means.     Am. 

Inst.  Elec.  Eng.,  25,  pp.  219,  473;  1906. 

LAWS  OF  RADIATION. 

EARLY    WORK. 

Newton.  —  Opuscula  Mathematica,  2,  p.  417. 

Prevost.  —  Sur  1'equilibre  du  feu,  Geneva,  1792,  1809. 

Dulong  and  Petit.  —  Ann.  Chim.  et  Phys.,  7,  pp.  225,  337;  1817. 

Kirchhojf.- — Pogg.  Ann.,  109,  p.  275;  1860.     Ann.  Chim.  et  Phys.,  69,  p.  124;  1860. 

B.  Stewart.  — •  Edinburgh  Trans.,  1858;  Proc.  Roy.  Soc.,  10,  p.  385;  1860. 
Prowstaye  and  Desains.  —  Ann.  Chim.  et  Phys.,  1860-1865. 

Draper.  —  Am.  Jour.  Sci.,  4,  p.  388;  1847. 
Becquerel.  —  C.  R.,  55,  p.  826;  1862. 
Clausius.  —  Pogg.  Ann.,  121,  p.  i;  1864. 

F.  Rosetti.  —  Phil.  Mag.  (5),  7,  pp.  324,  438,  537;  1879. 

Violle.  —  C.  R.,  88,  p.  171;  1879.     92*  PP-  866,  1204;  1881.     106,  p.  163;  1887. 

Weber.  —  Wied.  Ann.,  32,  p.  256;  1887. 

Tail.  —  Edinb.  Proc.,  12,  p.  531;  1884. 

Tyndall.  —  Phil.  Mag.,  28,  p.  329;  1864.  —  The  laws  of  radiation  and  absorption. 

Me"m.  by  Prevost,  Stewart,  Kirchhoff,  and  Bunsen,  edited  by  D.  B.  Brace. 

Am.  Bk.  Co.,  1902. 
Ritchie.  —  Mutual  radiation.     Pogg.  Ann.,  38,  p.  378;  1866. 

RECENT  WORK. 

Kayser.  —  Handbuch  der  spectroscopie,  2,  1902.     (Contains  an  admirable  sum- 
mary of  the  work  on  radiation.) 
Drude.  —  Theory  of  Optics  (Engl.  trans,  pub.  by  Longmans). 


472  HIGH  TEMPERATURES 

Stefan.  —  On  the  relation  between  heat  radiation  and  temperature.  Wien.  Ber., 
79,  B.  2,  p.  391;  1879. 

Schleirmacher.  —  On  Stefan's  law.     Wied.  Ann.,  26,  p.  287;  1885. 

Boltzmann.  — Deduction  of  Stefan's  law.     Wied.  Ann.,  22,  p.  291;  1884. 

Paschen.  —  On  the  emission  of  heated  gases.  Wied.  Ann.,  50,  p.  409;  1893.  51, 
p.  i;  1894.  52,  p.  209;  1894.  —  On  the  emission  of  solids.  Wied.  Ann.,  49, 
p.  50;  1893.  58,  p.  455;  1896.  60,  p.  662;  1897.  Astrophys.  Jour.,  2,  p.  202; 

1895.  —  On  black-body  radiation.     Wied.  Ann.,  60,  p.  719;  1897.     Berl.  Ber., 
p.  959;  1899.     Ann.  der  Phys.,  4,  p.  277;  1901.     6,  p.  646;  1901. 

Paschen  and  Wanner.  —  On  a  photometric  method,  etc.     Berl.  Ber.,  p.  5;  1899. 
Wanner.  —  Photometric  measurement  of  black- body  radiation.     Ann.  der  Phys., 

2,  p.  141;  1900. 

Fery.  —  Radiation  from  oxides.     Ann.  Chim.  Phys.  (7),  27,  p.  433;  1902. 
Petavel. — Heat  dissipated  by  platinum,  etc.     Phil.  Trans.,   191,  p.  501;  1898. 

197,  p.  229;  1901. 
Milliken.  —  Polarization  of  light  from  incandescent  surfaces.     Phys.   Rev.,   3, 

p.  177;  1895. 
Langley.  —  Distribution  of  energy  in  solar  spectrum,  etc.     Am.  Jour.  Sci.,  31, 

p.  i;  1886.     36,  p.  367;  1888.     Phil.  Mag.  (5),  26,  p.  505;  1888. 
Wilson  and  Gray.  —  Temperature  of  arc  and  sun.     Proc.  Roy.  Soc.,  66,  p.  250; 

1894.     68,  p.  24;  1895. 
W.  Michelson.  —  Theoretical  study  of  the  distribution  of  energy  in  the  spectra  of 

solids.     Jour,  de  Phys.  (2),  6,  p.  467;  1887.     Phil.  Mag.  (5),  25,  p.  425;  1888. 

Jour.  Russian  Phys.-chem.  Soc.,  34,  p.  155;  1902. 
Violle.  — The  radiation  of  incandescent  bodies.     Jl.  de  Phys.  (3),  1,  p.  298;  1892. 

C.  R.,  114,  p.  734;  116,  p.  1273,  1892.  —  Radiation  from  refractory  bodies 

heated  in  the  electric  furnace.     C.  R.,  117,  p.  33;  1893. 
St.  John.  —  On  the  equality  of  light  emissivities  at  high  temperatures,  etc.     Wied. 

Ann.,  66,  p.  433;  1895. 
Kurlbaum.  —  On  the  new  platinum  light  unit  of  the  Phys.  Tech.  Reichsanstalt. 

Verh.  Phys.  Ges.  (Berlin),  14,  p.  56;  1895. 
Larmor.  —  On  the  relations  of  radiation  to  temperature.     Nature,  62,  p.  562; 

1000.    63,  p.  216.  —  Planck's  eq.  Proc  Roy.  Soc.,  83,  p.  81;  1909.  —  Theory. 

Phil.  Mag.,  20,  p.  353;  1910. 
Guittaume.     -The  laws  of  radiation  and  the   theory  of  incandescent   mantles. 

Rev.  Ge"n.  des.  Sci.,  12,  pp.  358,  422;  1901. 
Wien.  —  Black- body  radiation  and  the  second  law  of  thermodynamics.     Berl.  Ber., 

P-   555   1893.  —  Temperature  and  entropy  of  radiation.     Wied.   Ann.,   62, 

p.   132;  1894.  —  The  upper  limit  of  wave  lengths  in  the  radiation  of  solid 

bodies,  etc.     Wied  Ann.,  46,  p.  633;  1893.    62,  p.  150;  1894.  —  On  the  partition 

of  energy  in  the  emission  spectrum  of  a  black  body.     Wied.  Ann.,  68,  p.  662; 

1896.  —  On  the  theory  of  radiation  from  a  black  body.     Ann.  der  Phys.,  3, 
P-  53o;  1900-     Paris  Cong.  Rpts.,  2,  p.  23;  1900. 

Wien  and  Lummer.  —  Method  of  demonstrating  the  radiation  law  for  an  abso- 
lutely black  body.     Wied.  Ann.,  66,  p.  451;  1895. 
Beckmann.  —  Black-body  radiation,  etc.     Thesis,  Tubingen,  1898. 
Lummer.  —  On  the  gray  glow  and  red  glow.     Wied.  Ann.,  62,  p.  13;  1897. — Radi- 


BIBLIOGRAPHY  473. 

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Phys.  (3),  2,  p.  157;  1901.  3,  p.  261;  1902. 

Lummer  and  Jahnke.  —  Ann.  der  Phys.,  3,  p.  283;  1900. 

Compau.  —  Radiation  laws.     Ann.  Chim.  Phys.  (7),  26,  p.  488;  1902. 

Lummer  and  Pringsheim.  —  The  radiation  from  a  black  body  between  100°  and 
1300°  C.  Wied.  Ann.,  63,  p.  395;  1897. — Distribution  of  energy  in  spectrum 
of  black  body.  Verh.  Deutsche  Phys.  Ges.,  1,  pp.  23  and  215;  1899.  —  Infra- 
red radiation.  Verh.  Deutsche  Phys.  Ges.,  2,  p.  163;  1900.  —  On  black  radi- 
ation. Ann.  der  Phys.,  6,  p.  192;  1901. — The  theoretical  radiation  scale,  etc., 
at  2300°  absolute.  Verh.  Deutsche  Phys.  Ges.  (5),  1,  p.  3;  1903. 

Lummer  and  Kurlbaum.  —  Electrically  treated  black  body  and  its  temperature 
measurement.  Verh.  Phys.  Ges.  (Berlin),  17,  p.  106;  1898.  Ann.  der  Phys., 
6,  p.  829;  1901. —  On  the  change  of  photometric  intensity  with  temperature. 
Verh.  Deutsche  Phys.  Ges.,  2,  p.  89;  1900. 

Pringsheim.  —  Deduction  of  Kirchhoff's  law.  Verh.  Deutsche  Phys.  Ges.,  3,  p.  81 ; 
1901. j —  Emission  from  gases.  Paris  Cong.  Rpts.,  2,  1900. 

Bottomley.  — Radiation  from  solids.     Phil.  Mag.  (6),  4,  p.  560;  1902. 

Planck.  —  Entropy  and  temperature  of  radiant  heat,  etc.  Ann.  der  Phys.,  1, 
p.  719;  1900.  Sitzber.  Berl.  Akad.,  1,  p.  440;  1899.  —  On  irreversible  radia- 
tion. Ann.  der  Phys.,  1,  p.  69;  1900. — On  a  bettering  of  Wien's  spectral 
equation.  Verh.  Deutsche  Phys.  Ges.,  2,  p.  202;  1900.  Ann.  der  Phys.,  4, 
p.  553;  1901.  —  Energy  distribution  in  normal  spectrum,  Verh.  D.  Phys. 
Ges.,  2,  p.  237;  1900.  —  Theory.  Ann.  der  Phys.,  31,  p.  758;  1910. 

Rayleigh.  —  Phil.  Mag.  (5),  49,  p.  539;  1900.     (6)  1,  p.  98;  1901. 

Thiesen.  —  Verh.  Deutsche  Phys.  Ges.,  2,  p.  65;  1900. 

Rubens.  —  Infra-red  radiation.     Wied.  Ann.,  69,  p.  576;  1899. 

Rubens  and  E.  Nichols.  —  Wied.  Ann.,  60,  p.  418;  1897. 

Rubens  and  Kurlbaum.  —  Experimental  verifications.  Berl.  Ber.,  p.  929;  1900. 
Ann.  der  Phys.,  4,  p.  649;  1901.  Astrophys.  JL,  14,  p.  335;  1901.  Ann.  der 
Phys.,  4,  p.  649;  1904. 

Maxwell.  —  Pressure  of  radiation.  Electricity  and  Magnetism,  chap,  on  Electro- 
magnetic theory. 

Bartoli.  —  Wied.  Ann.,  22,  p.  31;  1884. 

Galitzine.  — Wied.  Ann.,  47,  p.  479;  1892. 

Pellat.  —  JL  de  Phys.,  July,  1903. 

Lebedew.  —  Ann.  de  Phys.,  6,  p.  433;  1901.     Paris  Cong.  Rpts.,  2,  p.  133;  1900. 

E.  F.  Nichols  and  Hull.  —  Phys.  Rev.,  901  and  1903;  Astrophys.  Jl.,  17,  p.  315;  1903. 

Rayleigh.  —  Phil.  Mag.  (6),  3,  p.  338;  1902. 

Day  and  Van  Orstrand.  —  The  black  body  and  the  measurement  of  extreme  tem- 
peratures. Astrophys.  JL,  19,  p.  i;  1904. 

C.  Mendenhall  and  Saunders.  —  Astrophys.  Jl.,  13,  p.  25;  1901. 

Rase h.  —  On  the  photometric  determination  of  temperatures,  etc.  Ann.  der 
Phys.,  13,  p.  193;  1904. 

G.  W.  Stewart.  —  Spectral-energy  curves  of  black  body  at  room  temperature. 
Phys.  Rev.,  16,  p.  306;  1902.  17,  p.  476;  1903. 

Cotton.  —  Kirchhoff's  Law.  Rev.  Ge"n.  des  Sci.,  Feb.  15,  1899.  Astrophys.  JL,  9, 
p.  237;  1899. 


474  HIGH  TEMPERATURES 

Angstrom.  —  Gaseous  Absorption.     Ann.  der  Phys.,  6,  p.  163;  1901. 

Bahr.  —  Ibid.,  29,  p.  780;  1909. 

Valentiner.  —  Stefan's  law  and  gas  scale  to  1600°  C.     Ann.  der  Phys.  (4),  31, 

p.  275;  1910. 
V.  Wartenberg.  —  Emissive  powers  vs.  temperature.     Verb.  Phys.  Ges.,  12,  p.  121; 

1910. 

Holborn  and  Henning.  —  Emissive  powers.     Berl.  Akad.  Ber.,  p.  311;  1905. 
Burgess.  —  Radiation  from  Cu.     Bull.  Bureau  Standards,  6,  p.  in;  1909. 
Poynting.  —  Radiation  in  the  Solar  System.     Phil.  Trans.,  202,  p.  525;  1903. 
Fery.  —  Ann.  Chim.  et  Phys.,  17,  p.  267;  1909.     Rev.  Gen.  d.  Sci.,  Sept.,  1903. — 

Stefan's  law.     C.  R.,  148,  pp.  915,  1150;  1909.  —  Conical  Receivers.     C.  R., 

148,  p.  777;  1909.  —  Selective  Receivers.     C.  R.,  148,  p.  1043;  I9°9- 
Nernst.  —  From  Gases.     Phys.  Zs.,  6,  p.  777;  1904. 
Holborn  and  Henning.  —  Light  Emission  and  Melting  Points.     Berlin  Sitz.  Ber., 

12,  p.  311;  1905. 

Lucas.  —  Pt  Radiation.     Phys.  Zs.,  6,  p.  418;  1905. 
Burgess.  —  Pt  Radiation.     Bull.  Bureau  Standards,  1,  p.  443;  1905. 
Hertzsprung.  —  Radiation  and  Luminous  Equivalent.     Zs.  wiss.  Photographic,  4, 

p.  42;  1906. 
Jeans.  —  Theory.     Proc.  Roy.  Soc.,  76,  p.  545;  1905.     Phil.  Mag.,  12,  p.  57; 

1906.     17,  p.  229;  1909. 
Lummer  and  Pringsheim.  —  Welsbach  Mantle.     Phys.  Zs.,  7,   p.  89;   1906.- — 

Lorentz- Jeans  Formula.     Phys.  Zs.,  9,  p.  449;  1908. 
Swinburne.  —  Proc.  Phys.  Soc.,  20f  p.  33;  1906.     Engineering  (Lond.),  82,  p.  217; 

1906. 

Cantor.  —  Radiation  and  Doppler's  Principle.     Ann.  der  Phys.,  20,  p.  333;  1906. 
Thomson  (J.  /.). —  Electrical  Origin  of  Radiation.     Phil.  Mag.,  14,  p.  217;  1907. — 

Theory.     Phil.  Mag.,  20,  p.  238;  1910. 
Lorentz. — Theory.     N.  Cimento,  16,  p.  5;  1908. 

Richardson.  —  lonization  and  Temperature.     Phys.  Rev.,  27,  p.  183;  1908. 
Coblentz.  —  Radiation  Constants.     Bull.  Bureau  Standards,  5,  p.  339;  1909.  — 

Reflecting  Power  of  Metals.     Jl.  Frank.  Inst,  Sept.,  1910. 
Hagen  and  Rubens.  —  Emissivity  and  Temperature.      Sitz.    Berlin   Akad.,   16, 

p.  478;  1909.     23,  p.  467;  1910. 
H.  A.  Wilson.  — Theory.     Proc.  Roy.  Soc.,  82,  p.  177;  1909.     Phil.  Mag.,  20, 

p.  121;  1910. 
Pokrovzkij.  —  Spectrophotometric  Relations.     Jl.   Russ.    Phys.-chem.   Soc.,    41, 

p.  73;  1909. 

Polara.  —  Theory.     Accad.  Lincei,  18,  p.  513;  1909. 
Bauer  and  Moulin.  —  Stefan's  Law.      C.  R.,  149,  p.  988;  1909.      C.  R.,  160, 

p.  167;  1910. 

Einstein. — Theory.     Phys.  Zs.,  10,  p.  185;  1909. 
Saurel.  —  Theory.     Phys.  Rev.,  30,  p.  350;  1910. 
Fery  and  Drecq.  —  Stefan's  Law.    Jl.  de  Phys.  (5),  1,  p.  551;  1911. 
Parmentier.  —  Stefan's  Law.     Ann.  Ch.  et  Phys.,  22,  p.  417;  1911. 
Warburg   and   Leithauser.  —  Wien's   law.      Sitz.    Berlin    Akad.,    p.   925;    1910. 

P.  T.  R.  Thatigkeit,  1910. 


BIBLIOGRAPHY  475 

Humphreys.  —  Summary.     Astrophys.  Jl,  31,  p.  281,  1910. 
Violle.  —  Report  on  Radiation.     Ann.  Ch.  et  Phys.  (8),  2,  p.  134;  1904. 
Drysdale.  —  Luminous  Efficiency  of  Black  Body.     Jl.  de  Phys.   (4),  8,  p.   197; 
1909. 

RADIATION  PYROMETER. 

Violle.  —  Solar  radiation.     Ann.  Chim.  et  Phys.  (5),  20,  p.  289;  1877.     Jour,  de 

Phys.,  p.  277;  1876. 

Rosetti.  — Ann.  Chim.  et  Phys.,  17,  p.  177;  1879.     Phil.  Mag.,  18,  p.  324;  1879. 
Deprez  and  d'Arsonval.  —  Socie"te  de  Physique,  Feb.  5,  1886.  . 
Boys.  —  Radiomicrometer.     Phil.  Trans.,  180,  p.  159;  1887. 
Wilson  and  Gray.  —  Temperature  of  the  sun.     Phil.  Trans.,  186,  p.  361;  1894. 
Langley.  —  Bolometer.     Am.  Jour.  Sci.,  21,  p.  187;  1881.     31,  p.  i;  1886.     32, 

p.  90;  1886.     (4),  6,  p.  241;  1898.    Jour,  de  Phys.,  9,  p.  59. 
Terreschin.  —  Diss.    St.   Petersburg,    1898.     Jour.   Russ.   Phys.-chem.   Ges.,   29, 

pp.  22,  169,  277;  1897. 

Petavel.  —  Proc.  Roy.  Soc.,  63,  p.  403;  1898.     Phil.  Trans.,  191,  p.  501;  1898. 
Abbot.  —  Bolometer.     Astrophys.  Jour.,  8,  p.  250;  1898. 
Belloc.  —  Bolometer  errors.     L'e"clair.  61ec.  (5),  15,  p.  383;  1898. 
Scheiner.  —  Radiation  and  temperature  of  sun  (Leipzig),  1899. 
Warburg.  —  Temperature  of  sun.     Verh.  D.  Ges.  (i),  2,  p.  50;  1899. 
Fery.  —  The  measurement  of  high  temperatures  and  Stefan's  law.     C.  R.,  134, 

p.  977;  1902.     Jl.  de  Phys.,  Sept.,  1904. 

Rubens.  —  Sensitive  Thermopile.     Zs.  Instr'k.,  18,  p.  65;  1898. 
Ftry.  —  C.  R.,  134,  p.  977;  1902.  —  Spiral  Pyrometer.     Engineering  (Lond.),  87, 

p.  663;  1909.     Bull.  Soc.  Franc.  Phys.,  p.  186;  1907. 

Coblentz.  —  Radiation  Instruments.     Bull.  Bureau  Standards,  4,  p.  391;  1908. 
Foster.  —  Trans.  Am.  Electroch.  Soc.,  17,  p.  223;  1910. 
Angstrom.  —  Pyrheliometer.      Astrophys.  Jl.,  9,   p.    232.      Solar  Research  Int. 

Union,  1,  p.  178. 
Callendar.  —  Absolute  Bolometer.     Proc.  Roy.  Soc.,  77,  p.  6;  1905.     Phys.  Soc. 

Lond.,  May  12,  1905;  July  8,  1910. 

Kimball.  —  Pyrheliometers.     Bull.  Mt.  Weather  Obs.,  1,  p.  83;  1908. 
Abbot  and  Fowle.  —  Pyrheliometers  and  Bolometers.     Annals  Smithsonian  Astro- 
phys. Obs.,  2,  1908. 

Abbot.  —  Silver-disc  Pyrheliometer.     Smithsonian  Misc.  Coll.,  66,  p.  i;  1911. 
Thwing.  — JL  Frank.  Inst.,  May,  1908. 

OPTICAL  PYROMETER. 

See  also  laws  of  radiation  under  Lummer,  Pringsheim,  Kurlbaum,  Wanner,  Wien, 

Rasch,  Fe"ry,  etc. 

Kirehhojf.  — Ann.  Chim.  et  Phys.,  59,  p.  124;  1860. 
Ed.  Becquerel.  —  Optical  measurement  of  temperatures.     C.  R.,  66,  p.  826;  1863. 

Ann.  Chim.  et  Phys.,  68,  p.  49;  1863. 
Violle.  —  Radiation  from  platinum.     C.  R.,  88,  p.  171;  1879.     91,  PP-  866,  1204;. 

1881. 


476  HIGH  TEMPERATURES 

Kurlbaum  and  Schulze.  —  Pyrometric  examination  of  Nernst  lamps.  Verb.  D. 
Phys.  Ges.  (5),  24,  p.  425;  1903. 

D.  Berthdot.  —  On  a  new  optical  method,  etc.  Ann.  Chim.  et  Phys.  (7),  26, 
p.  58;  1902. 

Lummer.  —  Photometric  pyrometer.     Verb.  D.  Phys.  Ges.,  p.  131;  1901. 

Fery.  —  Temperature  of  the  arc.  C.  R.,  134,  p.  1201;  1902.  —  Absorption  Pyrom- 
eter. Jl.  de  Phys.  (4),  3,  p.  32;  1904. 

Le  Chatdier  Pyrometer.  —  On  the  measurement  of  high  temperatures.  C.  R., 
114,  pp.  214-216;  1892.  Jl.  de  Phys.  (3),  1,  pp.  185-205;  1892.  Industrie 
^lectrique,  April,  1892. — On  the  temperature  of  the  sun.  C.  R.,  114,  pp. 
737-739;  1892.  —  On  the  temperatures  of  industrial  furnaces.  C.  R.,  114, 
pp.  470-473;  1892.  Introduction  to  Metallurgy  (Roberts-Austen),  1903.  — 
i  Discussion  of  Le  Chatelier's  method:  (Violle)  C.  R.,  114,  p.  734;  1892.  Jl. 
de.  Phys.  (3),  1,  p.  298;  1892.  (Becquerd)  C.  R.,  114,  pp.  225  and  390;  1892. 
(Le  Chatdier)  C.  R.,  114,  p.  340;  1892.  (Crova)  C.  R.,  114,  p.  941;  1892. 
Kayser's  Spectroscopy,  1902. 

Crova' s  Pyrometer.  —  Spectrometric  study  of  certain  luminous  sources.  C.  R., 
87,  pp.  322-325;  1878.  —  On  the  spectrometric  measurement  of  high  tem- 
peratures. C.  R.,  87,  pp.  979-981;  1878.  —  Study  of  the  energy  of  radiations 
emitted  by  calorific  and  luminous  sources.  Jour,  de  Phys.,  7,  pp.  357-363, 
1878.  —  Spectrometric  measurement  of  high  temperatures.  Jour,  de  Phys.; 
9,  pp.  196-198;  1879.  C-  R.,  90,  pp.  252-254;  1880.  Ann.  Chim.  et  Phys. 
(5),  19,  pp.  472-550;  1880.  —  Photometric  comparison  of  luminous  sources 
of  different  lines.  C.  R.,  93,  pp.  512-513;  1881.  —  Solar  photometry.  C.  R., 
96,  pp.  1271-1273;  1882. 

End  of  Spectrum  Method.  —  A.  Crova.  Study  of  the  energy  of  radiations  emitted 
by  calorific  and  luminous  sources.  Jour,  de  Phys.,  7,  pp.  357-363;  1878. — 
W.  Hempel.  On  the  measurement  of  high  temperatures  by  means  of  a  spectral 
apparatus.  Zs.  f.  Angewandte  Chem.,  14,  pp.  237-242;  1901. 

Wanner  Pyrometer. — A.  Konig.  A  new  spectral  photometer.  Wied.  Ann.,  63, 
p.  785;  1894.  —  Description  of  Wanner  instrument.  Iron  Age,  Feb.  18, 
p.  24;  1904.  Phys.  Zs.,  3,  pp.  112-114;  1902.  Stahl  und  Eisen,  22,  pp.  207- 
211;  1902.  Zs.  Vereines  Deut.  Ing.,  48,  pp.  160,  161;  1904.  —  Wanner.  Pho- 
tometric measurement  of  the  radiation  of  black  bodies.  Ann.  der  Phys., 
6,  pp.  141-155;  1900. — Martens  and  Grinbaum.  Improved  form  of  Konig 
spectrophotometer.  Ann.  der  Phys.,  5,  p.  954;  1903.  —  Base.  Measurements 
with  Wanner  pyrometer.  Zs.  Anorg.  Chem.,  15,  p.  715;  1902.  — Nernst  and 
Wartenberg.  Verh.  Phys.  Ges.,  8,  p.  146;  1906. — Low- temperature  pyrometer. 
Jl.  Gasbel.,  60,  p.  1005;  1907. 

HolbornandKurlbatimPyrometer. — Sitzber.  Berlin Akad.,  June  13,  pp.  712-719;  1901. 
Ann.  der  Phys.,  10,  p.  225;  1902.    See  also  laws  of  radiation;  melting  points. 
Morse  Thermogage.  —  American  Machinist,  1903. 
Henning.  —  Spectral  pyrometer.     Zs.  Instr'kunde,  30,  p.  61;  1910. 
Waidner  and  Burgess.  —  Arc  Temperature.     Phys.  Rev.,  19,  p.  241;  1904.     Bull. 
Bureau  Standards,  1,  p.  109;  1904.  —  Optical  Pyrometry.     Phys.  Rev.,  19, 
p.  422;  1904.     Bull.  Bureau  Standards,  1,  p.  189;  1905.  —  Radiation  from  Pt 
and  Pd.    Bull.  Bureau  Standards,  3,  p.  163;  1907. 


BIBLIOGRAPHY  477 

Holborn.  —  Engineering  (Lond.),  84,  p.  345;  1907.     Brit.  Assoc.  Rpt,  p.  440;  1907. 

Nernst.  —  Photometric.     Phys.  Zs.,  7,  p.  380;  1906. 

Holborn  and  Valentiner.  —  Optical  vs.  Gas  to  1600°  C.     Ann.  der  Phys.  (4),  22, 

p.  i;  1907. 
Ffry.  —  Vacuum-tube  Temperatures.     Soc.  Franc.  Phys.,  p.  305;  1907.     Jl.  de 

Phys.,  6,  p.  979;  1907. 

Kurlbaum  and  Schulze.  —  Ibid.  Berichte  D.  Phys.  Ges.,  1906. 
Pirani.  —  Phys.  Ges.  Verh.,  12,  pp.  301,  1054;  1910.     13,  p.  19;  1911. 
C.  E.  Mendenhall.  —  Phys.  Rev.,  33,  p.  74;  1911. 
H.  W.  Gillett.—JL  Phys.  Chem.,  15,  p.  213;  1911. 
Thuermel.  — Ann.  de  Phys.,  33,  p.  1139;  1910. 
Wartenberg.  — Metal  temperatures.     Verh.  D.  Phys.  Ges.,  12,  p.  121;  1910. 

EXPANSION  AND  CONTRACTION  PYROMETERS. 

Wedgwood.  —  Phil.  Trans.,  72,  p.  305;  1782.     74,  p.  358;  1784. 

Weinhold.  —  Pogg.  Ann.,  149,  p.  186;  1873. 

Boh.  —  Die  Pyrometer,  Berlin,  1888. 

Guyton  and  Morveau.  — Ann.  Chim.  et  Phys.,  ist  Series,  46,  p.  276;  1803.  73, 
p.  254;  1810.  74,  pp.  18,  129;  1810.  90,  pp.  113,  225;  1814. 

The  Meldometer.  —  /.  Joly.  Proc.  Roy.  Irish  Academy  (3),  2,  p.  38;  1891.  —  Ram- 
say and  Eumorfopoulos.  Phil.  Mag.,  41,  p.  360;  1896. 

High-range  Mercury  Thermometers.  —  Weber.  Ber.  Berlin  Akad.,  Dec.,  1903. 
Wiebe,  ibid.,  July,  1884,  Nov.,  1885;  Zs.  Instr'kunde,  6,  p.  167;  1886.  8, 
P-  3735  l888-  10>  P-  2°7;  l89°-  Schott,  ibid.,  11,  p.  330;  1891. — Hovestadt. 
Jenaer  Glas  (trans,  by  J.  D.  and  A.  Everett).  — Marchis.  Modifications  per- 
manentes  du  verre,  etc.  —  Dickinson.  Bull.  Bureau  of  Standards,  2,  p.  189; 

1906.  —  Guillaume.   Treatise  on  Thermometry  of  Precision.    (Gauthier  Villars, 
1889.)  —  Fischer  and  Bobertag.     Glass  Therms,  for  High  Temps.     Zs.  f.  Elek. 
Ch.,  14,  p.  375;  1908.  —  Bureau  of  Standards.     Circ.,  8,  2nd  Ed.,  1911. 

Quartz  Thermometers. — Dufour.  C.  R.,  188,  p.  775;  1900. — Siebert.  Zs.  Elek- 
troch.  (Halle),  10,  p.  26. 

FUSIBLE-CONE   PYROSCOPE. 

Lauth  and    Vogt.  —  Pyrometric  measurements.     Bull.   Soc.    Chim.,   46,   p.    786; 

1886. 

Seger.  —  Tonindustrie  Zeitung,  p.  121;  1885.     Pp.  135,  229;  1886. 
Tonindustrie  Zs.,  No.  49,  1893;  No.  52,  1895;  No.  119,  1908. 
Stahl  u.  Eisen,  p.  440;  1909.     P.  1505;  1910. 
A.  Beranger.  —  Materiaux  et  Produits  Refractaires  (Paris,  1910).     (Very  complete 

account.)     Zs.  Angew.  Chem.,  p.  49;  1905.     Sprechsaal,  pp.  118,  156,  391,  483; 

1907.  P.  1284;  1906.     P.  561;  1908. 

Rothe.  —  Tonindustrie  Zs.,  30,  p.  1473;  1906.     31,  p.  1365;  1907. 
Hoffmann.  —  Tonindustrie  Zs.,  33,  p.  1577;  1909. 
Cramer  and  Hecht.  —  Tonindustrie  Zs.,  No.  18,  1896. 
Seger's  Schriften. 


47$  HIGH  TEMPERATURES 

Simonis.  —  Tonindustrie  Zs.,  31,  p.  146;  1907.     32,  p.  1764;  1908. 
H.  O.  Hofmann.  —  Trans.  Am.  Inst.  Mining  Eng.,  24,  p.  42.     Thatigkeit  P.  T. 
Reichsanstalt  for  1909,  1910. 

PYROMETERS  BASED  ON  FLOW  OF  FLUIDS. 

A.  Job.  —  Viscosity  pyrometer.     C.  R.,  134,  p.  39;  1902. 

U filing  and  Steinbart.  —  Stahl  u.  Eisen,  1899. 

Carnelly  and  Burton. — Water  circulation.    Jl.  Chem.  Soc.  (Lond.),  45,  p.  237; 

1884.     Also  described  in  Sir  Roberts-Austen's  Metallurgy,  1902. 
Barus.  —  Bull.  54,  Geolog.  Survey,  1889. 
E.  A.  Uehling.  —  Proc.  Cleveland  Inst.  Engrs.,  Jan.,  1900.    Electroch.  and  Met. 

Ind.,  3,  p.  160;  1905. 

RECORDING  PYROMETERS. 

Le  Chatelier.  —  Study  of  clays.     C.  R.,  104,  p.  144;  1887.  •«-  Quenching  of  Steels. 

Rev.  de  M6tallurgie,  1,  p.  134;  1904. 
Roberts- Austen.  —  First  Report  to  the  Alloys  Research  Committee.     Proc.  Inst. 

Mech.  Engrs.,  p.  543;  1891.     Nature,  45,  1892;  B.  A.  Report,  1891;  Jour,  of 

Soc.  of  Chem.  Industry,  46,  p.  i;  1896.     Proc.  Inst.  Mech.  Eng.,  p.  269;  1895. 

Pp.  67,  243;  1897.     Proc.  Roy.  Soc.,  49,  p.  347;  1891.     Fifth  Report  Alloys 

Research  Committee,  1899.     Proc.  Inst.  Mech.  Eng.,  p.  35;  1899.     Metal- 

lographist,  2,  p.  186;  1899. 
G.  Charpy.  —  Study  of  the  hardening  of  steel.      Bull,  de  la  Soc.  d'encouragement 

(4),  10,  p.  666;  1895. 
Callendar.  —  Platinum  recording  pyrometer.     Engineering,  May  26,  p.  675,  1899. 

Phil.  Mag.,  19,  p.  538;  1910. 
Stansfield.  —  Phil.  Mag.  (5),  46,  p.  59;  1898.     Phys.  Soc.  London  (2),  16,  p.  103; 

1898. 

Bristol.  —  Air  pyrometer.     Eng.  News,  Dec.  13,  1900. 
Saladin.  —  Iron  and  Steel  Metall.  and  Metallog.,  7,  p.  237;  1904. 
Queen  and  Co.  —  Electroch.  and  Met.  Ind.,  3,  p.  162;  1905. 
Siemens  and  Halske.  — Zs.  Instrumentenk.,  24,  p.  350;  1904.     25,  p.  273;  1905. 
Einthoven.  —  Arch.  Neerland,  10,  p.  414.     Proc.  Acad.  Sci.  Amsterdam  (2),  6,, 

p.  707;  1904. 

Benedicks.  —  Iron  and  Steel  Inst.,  II,  p.  153;  1908. 
Le  Chatelier.  —  Rev.  de  Metallurgie,  1,  p.  134;  1904. 
Wologdine.  —  Rev.  d.  Metallurgie,  4,  p.  552;  1907. 
Kurnakow.  —  Zs.  Anorg.  Ch.,  42,  p.  184;  1904. 
Schmidt.  —  Chem.  Eng.,  6,  p.  80;  1907. 
Harkhort.  —  Metallurgie,  4,  p.  639;  1907. 
Bristol.  — Trans.  Am.  Inst.  Mech.  Engrs.,  22,  No.  874,  p.  143. 
Dejean.  —  Rev.  de  M6tallurgie,  2,  p.  701;  1905.     3,  p.  149;  1906. 
Burgess.  —  Methods  of  Obtaining  Cooling  Curves.     Bull.  Bureau  Standards,  6,  p. 

199;  1908. 
Bruger.  —  Phys.  Zs.,  p.  775;  1906. 


BIBLIOGRAPHY  479 

Brown.  —  Electroch.  and  Met.  Ind.,  7,  p.  329;  1909. 
Northrup. —  Proc.  Am.  Electroch.  Soc.,  May,  1909. 
Rengade. — Bull.  Soc.  Chim.,  7,  p.  934;   1909. 
Hayes.  — Proc.  Am.  Acad.,  47,  p.  3;   1911. 

VARIOUS  PYROMETRIC  METHODS. 

Quincke.  —  Acoustic  Thermometer.     Ann.  d.  Phys.,  63,  p.  66;  1897. 

Krupp's  Hot-blast  Pyrometer.  —  Von  Bergen.    Jl.  Iron  and  Steel  Inst.,  1,  p.  207; 

1886. 
A.  H.  Sexton.  —  Fuel  and  Refractory  Materials.     Chapter  on  Pyrometry;  several 

industrial  forms  described. 

T.  Hurter.  —  Industrial  Air  Pyrometer.     Jl.  Soc.  Chem.  Ind.,  p.  634;  1886. 
WiborgWs  Pyrometer.  —  Jl.  Iron  and  Steel  Institute,  2,  p.  no;  1880. 
Fournier's  Vapor  Pressure  Thermometers.  —  Engineering  (Lond.)   89,   p.   447  ; 

1910. 

MELTING  POINTS. 

METALS. 

Prinsep.  —  Ann.  Chim.  et  Phys.  (2),  41,  p.  247;  1829. 

Lauth.  —  Bull.  Soc.  Chim.,  Paris,  46,  p.  786;  1886. 

E.  Becquerel.  — Ann.  Chim.  et  Phys.  (3),  68,  p.  497;  1863. 

Viotte.  —  C.  R.,  86,  p.  543;  1877.     87,  p.  981;  1878.     89,  p.  702;  1879. 

Holborn  and  Wien.  —  Wied.  Ann.,  47,  p.  107;  1892.     66,  p.  360;  1895.     Also  Zs. 

fur  Instr'k.,  p.  257;  1892. 
Holborn  and  Day.  —  Ann.  d.  Phys.  (i),  4,  p.  99;  1901.     Am.  Jour.  Sci.,  11,  p.  145; 

1901.     Wied.  Ann.,  68,  p.  817;  1899.     Am.  Jour.  Sci.  (4),  8,  p.  165;  1899. 
Ehrhardt  and  Schertel.  —  Jahrb.  fur  das  Berg-  und  Huttenw.  im  K.  Sachsen,  p.  154; 

1879. 

Callendar.  —  Phil.  Mag.  (5),  47,  p.  191;  1899.     48,  p.  519. 
Curie.  —  Ann.  de  Chim.  et  de  Phys.  (5),  6,  1895. 
Earns.  —  Bull.  54,  U.  S.  Geological  Survey,  1889.     Behandlung  u.  Messung  hoher 

Temp.  Leipzig,  1892.    Am.  Jour.  Sci.  (3),  48,  p.  332;  1894. 
Berthelot.  —  C.  R.,  126,  Feb.,  1898. 
Le  Chatelier.  —  C.  R.,  114,  p.  470;  1892. 

Moldenke.  —  Zs.  fur  Instr'k.,  19,  p.  153;  1898.     (Iron  and  steel.) 
Cusack.  —  Proc.  Roy.  Irish  Acad.  (3),  4,  p.  399;  1899. 
Landolt  and  Bornstein.  —  Phys.  Chem.  Tabellen,  Berlin. 
Carnelly.  —  Melting  and  Boiling  Point  Tables,  London,  1885. 
Smithsonian  Physical  Tables,  5th  ed.,  1910.  . 
Holman,  Lawrence,  and  Ban.  —  Phil.  Mag.  (5),  42,  p.  37;  1896.     Proc.  Am.  Acad., 

31,  p.  218. 
Heycock  and  Neville.  —  Phil.  Trans.,  189,  p.  25.     Jour.  Chem.  Soc.,  71,  p.  333; 

1897.     Nature,  66,  p.  502;  1897.     Chem.  News,  76,  p.  160;  1897. 
Heraus.  —  Manganese.     Zs.  Elecktroch.,  8,  p.  185;  1902. 
Nernst. — Zs.  Elektrotech.,  1903. 


480  HIGH  TEMPERATURES 

Rase h.  —  Ann.  d.  Phys.,  1904. 

D.  Berthdot.  —  Ann.  Chim.  et  Phys.,  1902.  —  Gold.     C.  R.,  138,  p.  1153;  1904. 

Richards.  —  Application  of  phase  rule  to  Cu,  Ag,  Au.     Am.  Jl.  Sci.  (4),  13,  p.  377; 

1902. 

Jaquerod  and  Perrot.  —  Gold.     C.  R.,  138,  p.  1032;  1904. 
Pirani  and  Meyer.  —  Ta.     Verh.  D.  Phys.  Ges.,  13,  p.  540;  1911. 
Fery  and  Cheneveau.  —  Pt.     C.  R.,  148,  p.  401;  1909. 
Carpenter.  —  Iron.     Iron  and  Steel  Inst.  III.,  p.  290;  1908. 
Lewis.  —  Cr.     Chem.  News,  86,  p.  13;  1902. 

Holborn  and  Henning.  —  Sn,  Cd,  Zn.     Ann.  d.  Phys.,  35,  p.  761;  1911. 
Day  and  Allen.  —  Phys.  Rev.,  19,  p.  177;  1904. 
Day  and  Clement.  —  Am.  Jl.  Soc.,  26,  p.  405;  1908. 
Day  and  Sosman.  —  Zn  to  Pd.     Am.  Jl.  Sci.,  29,  p.  93;  1910. 
Tammann  and  Associates  in  Zs.  Anorg.  Chem.  Metals  and  Binary  Alloys. 
Tammann.  —  Pressure  influence  on  Sn  and  Bi.     Zs.  Anorg.  Ch.,  40,  p.  54;  1904. 
Johnston  and  Adams. — Ibid.  Amer.  Jl.  Sci.,  31,  p.  501;   1911. 
Arndt.  —  Review.      Verein.  z.  Beford.  d.  Gewerbefleisses,  Verhandt.,  p.  265;  1904. 
Barker.  —  Pt  and  Ni.     Proc.  Roy.  Soc.,  76,  p.  235;  1905. 
Nernst  and  Wartenberg.  —  Pd  and  Pt.     Phys.  Ges.  Verh.,  8,  p.  48;  1906. 
Holborn  and  Valentiner.  —  Pd  and  Pt.     Sitzber.  Berlin  Akad.,  44,  p.  811;  1906. 
Waidner  and  Burgess.  —  Pd  and  Pt.     Bull.  Bureau  Standards,  3,  p.  163;  1907.  — 

Pt.     C.  R.,  May  3,   1909.  —  W  and  Ta.     Jl.  d.  Phys.,  6,  p.  830;  1907.— 

Reproducibility  of  Metal  M.P.'s.     Bull.  Bureau  Standards,  6,  p.  149;  1909. 
Burgess.  —  Iron  Group.     Bull.  Bureau  Standards,  3,  p.  345;  1907.  —  Chemical 

•Elements.     Jl.  Wash.  Acad.  Sci.,  1,  p.  16;  1911. 
Dejean.  —  Copper.     Rev.  d.  Metallurgie,  3,  p.  149;  1906. 
Mendenhall  and  Ingersoll.  —  Pd,  Rh,  Ir  on  Nernst  Glower.     Phys.  Rev.,  26,  p.  i; 

1907. 

Locke.  —  A  wire  method.     Zs.  Elektroch.,  13,  p.  592;  1907. 
Wartenberg.  —  W.     Chem.  Ber.,  40,  p.  3287;  1907.  —  Refractory  Metals.     Phys. 

Ges.  Verh.,  12,  p.  121;  1910. 

Ruff.  —  Refractory  Metals  and  Oxides.     Chem.  Ber.,  43,  p.  1564;  1910. 
Bolton.  —  Niobium.    Zs.  Elek.  Ch.,  13,  p.  145;  1907. 


SALTS    AMD    MISCELLANEOUS. 

Carnelly.—J.  C.  S.  Trans.,  p.  489;  1876.     P.  365;  1877.     P.  273;  1878. 

Le  Chatelier.  —  Bull.  Soc.  Chim.,  47,  p.  301.     C.  R.,  118,  pp.  350,  711,  802. 

V.  Meyer,  Riddle,  and  Lamb.  —  Ch.  Ber.,  27,  p.  3129;  1894. 

MacCrae.  —  Ann.  d.  Phys.,  66,  p.  95;  1894. 

Hutner  and  Tammann.  — Zs.  Anorg.  Ch.,  43,  p.  215;  1905. 

Ruff  and  Plato.  —  Ch.  Ber.,  36,  p.  2357;  1903. 

Joly.  —  Proc.  Irish  Acad.,  No.  2,  1891. 

Ramsay  and  Eurmorfopoulos.  —  Phil.  Mag.,  41,  p.  360;  1896. 

Day  and  Sosman.  —  Am.  Jl.  Sci.,  31,  p.  341;  1911. 

Grenet.  —  Rev.  de  Metallurgie,  7,  p.  485;  1910. 


BIBLIOGRAPHY  481 

Brearly  and  Morewood.  —  Sentinel   Pyrometers.     Jl.   Iron  and  Steel   Inst.,   73, 

p.  261;  1907. 

Liebknecht  and  Nilsen.  —  Ch.  Ber.,  36,  p.  3718;  1903. 
Watson.  —  Phys.  Rev.,  26,  p.  198;  1908. 

C.  H.  Burgess  and  A.  Holt.  —  Rapid  for  borates,  etc.     Proc.  Roy.  Sri.  Lond., 

Nov.  24,  1904. 

W.  C.  Herceus.  —  Ceramics.     Zs.  Angew.  Chem.,  p.  49;  1905. 
Doelter.  —  Silicates.     Zs.  Elektroch.,  12,  p.  617;  1906. 
Lampen.  —  Porcelains,  etc.     Jl.  Am.  Ch.  Soc.,  28,  p.  846;  1906. 
White.  —  Thermoelectric  manipulation.     Am.  Jl.  Sci.,  28,  p.  453;  1909. 
Boudouard.  —  Slags.     Jl.  Iron  and  Steel  Inst.  (i),  67,  p.  350;  1905. 

BOILING  POINTS. 

Earns.  —  (See  Melting  Points.)     Am.  Jour.  Sci.  (5),  48,  p.  332;  1894. 

Troost.  —  C.  R.,  94,  p.  788;  1882.     94,  p.  1508;  1882.    96,  p.  30;  1882. 

Le  Chatelier.  —  C.  R.,  121,  p.  323;  1895.     (See  also  under  Thermoelectric  Pyrom- 
eter.) 

Berthelot.  —  Stances  de  la  soc.  de  physique,  Paris,  Feb.,  1898,  and  Bull,  du  Museum, 
No.  6,  p.  301;  1898. 

Callendar  and  Griffiths. —  Proc.  Roy.  Soc.  London,  49,  p.  56;  1891. 
>Chappuis  and  Barker.  —  Travaux  et  M6m.  du  Bureau  Int.  des  Poids  et  Mesures, 
12,  1900;  Phil.  Trans.,  1900. 

Preyer  and  V.  Meyer. — Zeits.  fur  Anorg.  Chem.,  2,  p.  i;  1892.     Berl.  Ber.,  25, 
p.  622;  1892. 

S.  Young.  —  Trans.  Chem.  Soc.,  p.  629;  1891. 

MacCrae. — Wied.  Ann.,  66,  p.  95;  1895. 

Callendar.  —  Phil.  Mag.  (5),  48,  p.  519;  1899.     (Fusion  also.) 

D.  Berthelot.  —  Ann.  Chim.  et  Phys.,  1902. 

Fery.  —  Cu  and  Zn.     Ann.  Chim.  et  Phys.  (7),  28,  p.  428;  1903. 

R.  Rothe.  —  Sulphur.     Zs.  Instrumk.,  23,  p.  364;  1903. 

Greenwood.  —  Of  metals.      Proc.  Roy.  Soc.,  A  82,  p.  396;  1909.     83,  p.  483;  1910. 

Chem.  News,  104,  pp.  31,  42;   1911. 

Kraft  and  Merz.  —  S,  Se,  Te  at  reduced  pressure.      Chem.  Ber.,  36,  p.  4344J  1903- 
Ruff  and  Johannsen.  —  Alkali  metals.     Chem.  Ber.,  38,  p.  3601;  1905. 
Harker  and  Sexton.  —  Pressure  change  of  S.B.P.     Elect.  Rev.,  63,  p.  416;  1908. 
Eumorfopoulos.  —  Sulphur.     Proc.  Roy.  Soc.,  81,  p.  339;  1908. 
Callendar  and  Moss.  —  Sulphur.     Proc.  Roy.  Soc.,  83,  p.  106;  1909. 
Holborn  and  Henning.  —  Sulphur,  etc.     Ann.  d.  Phys.  (4),  35,  p.  761;  1911. 
Waidner  and  Burgess.  —  Sulphur.     Bull.  Bureau  Standards,  7,  p.  127;  1911. 
Wartenberg.  —  Metals.     Zs.  Anorg.  Chem.,  66,  p.  320;  1908. 
Moissan.  —  Metals.     Ann.  Ch.  et  Phys.  (8),  8,  p.  145;  I9°6- 
Watts.  —  Metals.     Trans.  Am.  Electroch.  Soc.,  12,  p.  141;  1907. 
Smith  and  Menzies.  —  Hg.     Jl.  Am.  Ch.  Soc.,  32,  p.  1434;  1910. 
Jaquerod  and  Wassmer.  —  Benzophenone  and  Naphthaline.      Jl.   d.   Ch.  et  de 

Phys.,  2,  p.  52;  1904. 
Waidner  and  Burgess.  —  Ibid.     Bull.  Bureau  Standards,  7,  p.  3;  1910. 


482  HIGH  TEMPERATURES 

PYROMETRIC    MATERIALS. 
PORCELAIN:  EXPANSION. 

Deville  and  Troost.  —  C.  R.,  67,  p.  867;  1863. 

Bedford.  —  B.  A.  Report,  1899. 

Benefit.  —  Trav.  et  Mem.  du  Bureau  Int.,  6,  p.  190. 

Tutton.  —  Phil.  Mag.  (6),  3,  p.  631;  1902. 

Chappuis.  —  Phil.  Mag.  (6),  3,  p.  243;  1902. 

Holborn  and  Day.  —  Ann.  der  Phys.  (4),  2,  p.  505;  1900. 

Holborn  and  Gruniesen.  —  Ann.  der  Phys.  (4),  6,  p.  136;  1901. 

METALS:  EXPANSION. 

Holborn  and  Day.  —  Ann.  der  Phys.,  4,  p.  104;  1901.     Am.  Jl.  Sci.  (4),  11,  p.  374; 

1901. 
Le  Chatelier.  —  C.  R.,  128,  p.  1444;  1899.     129,  p.  331.     107,  p.  862;  1888.     108, 

p.  1046;  1896.     Ill,  p.  123;  1890. 
Charpy  and  Grenet.  —  C.  R.,  134,  p.  540;  1902. 
Terneden. — Thesis,  Rotterdam,  1901  (Fortsch.  der  Phys.,  1901). 
Dittenberger.  — Zs.  Ver.  Deutsche  Ingen.,  46,  p.  1532;  1902. 
Day  and  Sosman.  —  Am.  Jl.  Sci.,  29,  p.  in;    1910. 

QUARTZ. 

Le  Chatelier.  —  C.  R.,  107,  p.  862;  1888.     108,  p.  1046;  111,  p.  123;  130,  p.  1703. 

Callendar.  —  Chem.  News,  83,  p.  151;  1901. 

Holborn  and  Henning.  —  Ann.  der  Phys.,  4,  p.  446;  1903. 

Scheel.  —  Deutsch.  Phys.  Ges.  (5),  5,  p.  119;  1903.  —  Verb.  Phys.  Tech.  Reich- 

sanstalt,  1904. 
Shenstone.  —  Properties  of  Amorphous  Quartz.     Nature,  64,   pp.   65  and   126; 

1901.     Contains  history  to  date. 

Dufour.  —  Tin-quartz  thermometer.     C.  R.,  130,  p.  775. 
Villard.  —  Permeability  for  H  at  1000°  C.     C.  R.,  130,  p.  1752. 
Joly.  —  Plasticity,  etc.     Nature,  64,  p.  102;  1901. 
Moissan  and  Siemens.  —  Action   of   water   on.     C.  R.,   138,  p.    939;    1904. — 

Solubility  in  Zn  and  Pb.     C.  R.,  138,  p.  86;  1904.  —  Vapor  pressure  of.     C.  R., 

138,  p.  243;  1904. 

Heraus.  —  Properties:  a  general  summary.     Zs.  Elektroch.,  9,  p.  848;  1903. 
Brun.  —  Fusion.     Arch.  Sc.  Phys.  Nat.  (Geneva)  (4),  13,  p.  313;  1902. 
Hutton.  —  Lamps,  etc.     Am.  Electroch.  Soc.,  Sept.,  1903. 
Day  and  Shepherd.  —  Science,  23,  p.  670;  1906. 

GLASS:  EXPANSION. 

Holborn  and  Gruniesen.  —  Ann.  der  Phys.  (4),  6,  p.  136;  1901. 
Bottomley  and  Evans.  —  Phil.  Mag.,  1,  p.  125;  1901. 
Hovestadt.  —  Jenaer  Glas. 


BIBLIOGRAPHY  483 

REFRACTORIES. 

(See  also  Furnaces.) 

E.  K.  Scott.  —  Furnace  linings.  Faraday  Soc.,  1905.  Electroch.  and  Met.  Ind., 
3,  p.  140;  1905. 

A.  Beranger.  —  Materiaux  et  Produits  Refractaires  (Paris,  1910). 

Neumann.  —  Stahl  u.  Eisen,  30,  p.  1505;  1910.  (Bibliography  of  alumina-lime- 
silica.) 

Fitzgerald.  —  Furnace  materials.     Electroch.  Ind.,  2,  p.  439,  490;  1904. 

Dunn.  —  Soc.  Ch.  Ind.  Jl.,  23,  p.  1132;  1904. 

Brauner.  —  Bibliography  of  Clays  and  the  Ceramic  Arts.  American  Ceramic 
Society. 

Hutton  and  Beard.  —  Conductivity.     Faraday  Soc.  Trans.,  1,  p.  264;  1905. 

Wologdine.  —  Conductivity,  etc.     Bull.  Soc.  d'Encour.,  8,  p.  879;  1909. 

Schoen.  —  Use  of  Quartz  Tubes.     Metallurgie,  6,  p.  635;  1908. 

Walden.  —  Jl.  Am.  Ch.  Soc.,  30,  p.  1351;  1908. 

Acheson.  —  Siloxicon.     Electroch.  Ind.,  6,  p.  379;  1908. 

Arndt.  —  Berlin  Magnesia.     Chem.  Ztg.,  30,  p.  211;  1906. 

Goodwin  and  M alley.  —  Phys.  Rev.,  23,  p.  22;  1906. 

Aubrey.  —  Bauxite.     Electroch.  and  Met.  Ind.,  4,  p.  52;  1906. 

Bleninger.  —  Fire  Clays.     Proc.  Eng.  Soc.  West.  Penn.,  25,  p.  565;  1910. 

Bywater.  —  Jl.  of  Gas  Lighting,  102,  p.  831;  1908. 

Boiling.  —  Silundum.     Electroch.  Ind.,  7,  p.  24;  1909. 

Tucker.  —  Ibid.,  7,  p.  512;  1909. 

Buchner.  —  Fused  Alumina.     Zs.  Anorg.  Ch.,  17,  p.  985;  1904. 

Phalen.  —  Alundum.     Electroch.  Ind.,  7,  p.  458;  1909. 

Day  and  Shepherd.  —  Lime-silica.     Am.  Jl.  Sci.,  22,  p.  265;  1906. 

.Shepherd  and  Rankine.  —  Binary  Systems.     Am.  Jl.  Sci.,  28,  p.  293;  1909. 

Heinicke.  —  Magnesia-alumina.     Zs.  Anorg.  Ch.,  21,  p.  687;  1908. 

Iddings.  —  Igneous  Rocks.     1909.     (Wiley  &  Sons.) 

Moissan.  —  Le  Four  Electrique.     (Also  in  English.) 

.Sexton.  —  Fuel  and  Refractory  Materials. 

VARIOUS   SUBJECTS. 

Le  Chatelier.  —  Specific  heat  of  carbon.     C.  R.,  116,  p.  1051;  1893.     Soc.  Franc. 

de  Phys.,  No.  107,  p.  3;  1898. 
Barus.  —  Bull,  of  U.  S.  Geological  Survey  No.  54,  1889.     (Pyrometry.)     Report 

on  the  progress  of  pyrometry  to  the  Paris  Congress,  1900.  —  Viscosity  and 

temperature.      Wied.  Ann.,    96,    p.    358;    1899.     Callendar.  —  Nature,  49, 

p.  494;  1899.  —  Long-range  temperature  and  pressure  variables  in  physics. 

Nature,  66,  p.  528;  1897. 

Baly  and  Charley.  —  Liquid-expansion  pyrometer.     Berl.  Ber.,  27,  p.  470;  1894. 
Dufour.  —  Tin  in  quartz  pyrometer.     C.  R.,  180,  p.  775;  1900. 
Berlhelot.  —  Interference    method    of    high-temperature    measurements.     C.    R., 

120,  p.  831;  1895.     Jour,  de  Phys.  (3),  4,  p.  357;  1895.     C.  R.,  Jan.,  1898; 

applications  in  C.  R.,  Feb.,  1898. 


484  HIGH  TEMPERATURES 

Moissan.  —  Le  Four  Electrique,  Paris,  1898.     (Also  in  English.) 

Tijpler.  —  Pressure-level  apparatus.    Wied.  Ann.,  56,  p.  609;  1895.     67,  p.  311; 

1896. 
Quincke.  —  An  acoustic  thermometer  for  high   and   low   temperatures.     Wied. 

Ann.,  63,  p.  66;  1897. 

K.  Scheel.  —  Ueber  Fernthermometer.     Verlag  v.  C.  Marhold,  Halle,  1898.     48  pp. 
Heitmann.  —  Ueber   einen  neuen  Temperatur-Fernmessapparat   von  Hartmann 

und  Braun.     E.  T.  Z.,  19,  p.  355;  1898. 

Chree.  —  Recent  work  in  thermometry.     Nature,  68,  p.  304;  1898. 
Lemeray.  —  On  a  relation  between  the  dilation  and  the  fusing  points  of  simple 

metals.     C.  R.,  131,  p.  1291;  1900. 
Holborn  and  Austin.  —  Disintegration  of  the  platinum  metals  in  different  gases. 

Phil.  Mag.  (6),  7,  p.  388;  1904. 

Stewart.  —  (Same  as  preceding.)     Phil.  Mag.  (5),  48,  p.  481;  1899. 
Hagen  and  Rubens.  —  On  some  relations  between  optical  and  electrical  prop- 
erties of  metals.     Phil.  Mag.  (6),  7,  p.  157;  1904. 
Kahlbaum.  —  On  the  distillation  of  metals.     Phys.  Zs.,  p.  32;  1900. 
Kahlbaum,  Roth,  and  Seidler.  —  Ibid.     Zs.  Anorg.  Ch.,  29,  p.  177;  1902. 
*  Glaser.  —  Latent  and  sp.  hts.  of  metals.     Me'tallurgie,  1,  pp.  103,  121;  1904. 
Richards.  —  Metallurgical  calculations. 
Scudder.  —  Liquid  baths  for  melting-point    determinations.      Chem.  News,  88r 

p.  104;  1903. 

Guertler.  —  Devitrification  temperatures.     Zs.  Anorg.  Ch.,  40,  p.  268;  1904. 
Kraft  and  Bergfeld.  —  Lowest  evaporation  temperatures  of  metals  in  vacuo.     Chem. 

Ber.,  38,  p.  254;  1905. 

Kraft.  —  Boiling  metals  in  vacuo.     Chem.  Ber.,  38j  p.  262;  1905. 
Wiebe.  —  Melting  Points  vs.  Expansion.     Phys.  Ges.  Verh.,  8,  p.  91;  1906. 
/.  W.  Richards.  —  Latent  Heat  of  Vaporization,  Metals,  etc.     Am.  Electroch.. 

Soc.  Trans.,  13,  p.  447;  1908. 

F.  C.  Mason.  —  Magnetic  Pyrometer.     Am.  Machinist,  33,  p.  875. 
Pawlow.  —  Melting  vs.  Surface  Tension.      Zs.  Phys.  Ch.,  66,  p.  i;  1908. 
Hot-blast  pyrometers.     Roberts- Austen's  Metallurgy.    J.   Iron  and   Steel  Inst., 

pp.  195,  240;  1884.    P.  235;  1885.    P.  207;  1886.    2,  p.  no;  1888.    Proc.  Inst. 

M.  E.,  p.  53;  1852.    Jl.  Soc.  Chem.  Ind.,  p.  40;  1885.    P.  16;  1897. 
Wiborgh.  —  Industrial  Air  Pyrometer.    Jl.  Ir.  and  St.  Inst.,  2,  p.  no;  1888. 
Callendar.  —  Industrial  Air  Pyrometer.      Proc.  Roy.  Soc.,  50,  p.  247;  1892. — 

Measurement  of  extreme  temperatures.     Nature,  69,  pp.  495  and  519  —  a 

review  of  various  pyrometric  methods. 

Siebert.  —  Quartz  Thermometers.     Zs.  Elektroch.  (Halle),  10,  p.  26. 
Mahlke.  —  On  a  comparison  apparatus  for  thermometers  between  250°  and  600°  C. 

Zs.  Instr'kunde,  14,  p.  73;  1894. 
F.  Kraft.  —  Evaporation  and  boiling  of  metals  in  quartz  in  electric  furnace.    Ber. 

Deut.  Ch.  Ges.,  36,  p.  1690;  1903. 


BIBLIOGRAPHY  485 

CHEMICAL  DETERMINATIONS  OF  TEMPERATURES. 

Haber  and  Richardt.  —  The  water-gas  equilibrium  in  the  Bunsen  flame,  and  the 

chemical  determination  of  high  temperatures.     Zs.  Anorg.  Chem.,  38,  p.  5; 

1904. 
Zenghelis.  —  Chemical  reactions  at  very  high  temperatures.     Zs.  Phys.  Chem., 

46,  p.  287 ;  1903. 
Nernst.  —  On  the  determination  of  high  temperatures.     Phys.  Zs.,  4,   p.   733; 

1903. 

THERM  OCHEMICAL    DATA. 

Haber.  —  Technical  Gas  Reactions,  1908.     (Tr.  by  A.  B.  Lamb.) 

Nernst.—  Q  =  RT2  (d\ogK/dT).     Phys.  Zs.,  4,  p.  733;  1903. 

Haber  and  Associates.  —  Gas  Combustion  Phenomena.  Zs.  Phys.  Chem.,  68, 
p.  726.  69,  p.  337;  1909.  71,  p.  29;  1910. 

Zenghelis. — Zs.  Phys.  Chem.,  46,  p.  287;  1903. 

Wartenberg.  — Reviews.     Fortschritte  d.  Chemie,  2,  p.  205;  1910. 

V.  Jiiptner.  —  Energy  relations.     Zs.  Anorg.  Ch.,  42,  p.  235;  1904. 

Wartenberg.  —  Phys.  Ges.  Verb.,  8,  p.  97;  1906. 

Nernst  and  Wartenberg.  —  H2O  and  CO2  dissociation.  Zs.  Phys.  Chem.,  66,  pp. 
5i3,  534,  548;  1906. 

Tucker  and  Lampen.  —  Carborundum  Formation  Temperatures.  Jl.  Am.  Chem. 
Soc.,  28,  p.  853;  1906. 

Button  and  Petavel.  —  High-temperature  Electrochemistry.  (References.)  Elec- 
trician, 50,  pp.  308,  349;  1902.  Phil.  Trans.,  207,  p.  421;  1908. 

INCANDESCENT-LAMP    TEMPERATURES. 

Le  Chatelier.  —  Jl.  d.  Phys.  (6),  1,  p.  203;  1892. 

Lummer  and  Pringsheim.  —  Verb.  D.  Phys.  Ges.,  1,  pp.  23,  215;  1899. 

Janet.  —  C.  R.,  123,  p.  690;  1896.     123,  p.  734;  1898. 

Pirani.  —  Verb.  Phys.  Ges.,  12,  p.  301;  1910. 

Fery  and  Cheneveau.  —  C.  R.,  149,  p.  777;  1909.     Jl.  de  Phys.,  9,  p.  397;  1910. 

Waidner  and  Burgess.  —  Bull.  Bureau  Standards,  2,  p.  319;  1907.     Elec.  Wld. 

48,  p.  915;  1906. 

Henning.  —  Zs.  Instr'kunde,  30,  p.  61;  1910. 
Grau.  —  Elektrotech.  u.  Maschinenb.,  25,  p.  295;  1907. 
Coblentz.  — Bull.  Bureau  Standards,  5,  p.  339;  1908. 
Morris,  Stroud,  and  Ellis.  —  Electrician,  59,  pp.  584,  624;  1907. 
Jolley.  —  Electrician,  63,  pp.  700,  755;  1909. 

TEMPERATURE    OF    FLAMES. 

Kurlbaum.  —  Phys.  Zs.,  3,  p.  187;  1902. 

Lummer  and  Pringsheim.  —  Phys.  Zs.,  3,  p.  233;  1902. 

E  W.  Stewart.  —  Phys.  Rev.,  1902,  1903. 

Fery.  —  C.  R.,  137,  p.  909;  1903. 

Haber  and  Richardt.  —  Zs.  Anorg.  Chem.,  38,  p.  5;  1904. 


486  HIGH  TEMPERATURES 

Haber  and  Hodsman.  —  Zs.  Phys.  Chem.,  67,  p.  343;  1909. 
Becker.  —  Ann.  der  Phys.,  28,  p.  1017;  1909. 
Allner.  —  Jl.  fur  Gasbelencht,  48,  pp.  1035,  1057,  1081,  1107; 
Amerio.  —  Accad.  Sci.  Torino,  41,  p.  290;  1905. 
Ladenburg.  —  Phys.  Zs.,  7,  p.  697;  1906. 
Schmidt.  —  Phys.  Ges.  Verh.,  11,  p.  87;  1909. 

Shea.  —  Bunsen  with  Thermocouples.     Phys.  Rev.,  30,  p.  397;  1910. 
E.  Bauer.  —  Le  Radium.     Dec.  6,  1909.     C.  R.,  147,  p.  1397;  1908.     C.  R.,  148, 
pp.  908,  1756;  1909- 

SOLAR    AND    STELLAR    TEMPERATURES. 

(See  also  Radiation  Pyrometer.) 

Abbot. — The  Sun,  ign.     Astro,  phys.  Jl.  34,  p.  197;  1911. 

Pringsheim.  —  Physik  der  Sonne,  1910. 

Goldhammer.  — Ann.  d.  Phys.,  25,  p.  905;  1908. 

Millochau.  —  C.  R.,  148,  p.  780;  1909.     Jl.  de  Phys.,  8,  p.  347;  1909. 

W Using  and  Scheiner.  —  Pub.  Astrophys.  Obs.,  Potsdam,  No.  56,  19.  Astrophys. 
JL,  32,  130;  1910. 

Abbot  and  Fowle.  —  Pub.  Astrophys.  Obs.  Smithsonian  Institution,  2,  1908.  Astro- 
phys. Jl.,  29,  p.  281;  1909. 

.Nordmann.  —  C.  R.,  149,  pp.  557,  662,  1038;  1909.  160,  pp.  448,  669;  1910. 
Bull.  Astron.,  Apr.,  1909,  p.  170. 

Kurlbaum.  —  Sitzber.  Berlin  Akad.,  25,  p.  541;  1911. 

LABORATORY    FURNACES. 

A.  Kalahne.  —  On  electric  resistance  furnaces.     Ann.  d.  Phys.,  11,  p.  257;  1903. 

E.  Haagen.  —  Platinum-foil  furnaces.     Zs.  Elektroch.,  p.  509;  1902. 

W.  C.  HercBUs. —  Electrical  laboratory  furnace.      Zs.  Elektroch.,  p.   201;  1902. 
^       Electrician,  49,  p.  519.      50,  p.  173;  1902. 

C.  L.  Norton.  —  Laboratory  electric  furnaces.     Elec.  World  and  Eng.,  36,  p.  951; 

1900. 

F.  A.  J.  Fitzgerald.  —  Principles  of  resistance  furnaces.     Trans.  Am.  Elec.  Chem. 

Soc.,  4,  p.  9;  Elec.  chem.  Indus.,  2,  p.  242;  1904. 

D.  Berthelot.  —  Ann.  Phys.  et  Chim.,  26,  p.  58;  1902. 
Holborn  and  Day.  —  Ann.  der  Phys.,  2,  p.  505;  1900. 

Doelter.  — Electric  furnaces  for  melting  points.     Centralbl.  f.  Min.,  p.  426;  1902. 
Day  and  Allen.  —  Phys.  Rev.,  19,  p.  177;  1904. 
King.  —  Carbon  vacuum  tube.     Astrophys.  Jl.,  28,  p.  300;  1908. 
Tucker.  — Tube  furnace.     Tr.  Am.  Electroch.  Soc.,  11,  p.  303;  1907. 
Lampen.—J\.  Am.  Ch.  Soc.,  28,  p.  846. 
Stansfield.  —  The  Electric  Furnace  (McGraw-Hill  Co.). 
H.  Moisan.  —  Le  Four  Electrique.     (Also  in  English.) 
J.  Wright.  —  Electric  Furnaces  and  Their  Application.     (Henley  Pub.  Co.) 
Hutton  and  Petavel.  —  Inst.  Elec.  Eng.  (Manchester  Sec.),  Nov.  25,  1902.    JL 
of,  32,  p.  227;  1903. 


BIBLIOGRAPHY  487 

Button  and  Patterson.  —  Electroch.  and  Met.  Ind.,  p.  455;  1905.     Trans.  Faraday 

Soc.,  1,  p.  187. 

Button.  —  Electrician,  68,  p.  579;  1907. 

Howe.  —  Electric  muffle.     Proc.  Am.  Soc.  Test.  Materials,  6,  p.  202;  1906. 
Friedrich.  —  Gas  metallographic  furnace.     Metallurgie,  3,  p.  206;  1906.  —  Electric 

furnaces,  Metallurgie,  4,  p.  778;  1907.     Jl.  Am.  Ch.  Soc.,  28,  p.  921;  1906. 
Arsem.  —  Vacuum  Furnace.     Jl.  Am.  Chem.  Soc.,  28,  p.  921;  1906.     Trans.  Am. 

Electroch.  Soc.,  9,  p.  153;  1906.     Jour.  Eng.  Chem.,  Jan.,  1910. 
Ruff.  — Vacuum  Furnace.     Ch.  Ber.,  43,  p.  1564;  1910. 
V.  Wartenberg.  —  Tungsten  furnace.    Zs.  Elek.  Ch.,  16,  p.  876;  1909. 
Heraus.  —  Iridium  Furnace.     Zs.  Angew.  Chem.,  18,  p.  49;  1905. 
Of  Kriptol:     Zs.  Angew.  Chem.,  18,  p.  239;  1905.      Metallurgie,  4,  pp.  617,  778; 

1907.     6,  pp.  186,  638;  1908. 

Mutter.  —  Vacuum  Furnace.     Metallurgie,  6,  p.  145;  1909. 
Oberho/er.  —  Vacuum  Furnace.     Metallurgie,  4,  p.  427;  1907. 
Sabersky-Adler.  —  Electric  hardening  furnace.      Trans.  Faraday  Soc.,  6,  p.  15; 

1909. 
Barker.  —  Solid  Electrolyte  Tube.     Proc.  Roy.  Soc.,  76,  p.  235;  1905. 

THERMOSTATS    AND    FURNACE    CONTROL. 

B.  Darwin.  —  Electric  Thermostat.     Electrician,  62,  p.  256.     Astrophysical  JL, 

20,  p.  347;  1904- 
Morris,  Ellis,  and  Stroud.  —  Automatic  Rheostat  Control.     Electrician  (Lond.), 

61,  p.  400;  1908. 

Sodeau.  —  Regulators.     Soc.  Chem.  Ind.  Jl.,  23,  p.  1134;  1904. 
Plato.  —  Mechanical  automatic  rheostat.     Zs.  Phys.  Ch.,  66,  p.  721;  1906. 
Portevin.  —  Continuous  water  rheostat.     Rev.  de  Metallurgie,  6,  p.  295;  1908. 
Kolowrat.  —  Electro-optical  regulator.     Jl.  d.  Phys.,  8,  p.  495;  1909. 
Bodenstein.  —  Thermostats.     Faraday  Soc.,  May  23,  1911. 

METALLOGRAPHIC   PRACTICE. 

Frankenheim.  —  Cooling  curves.     Pogg.  Ann.,  39,  p.  376;  1836. 

Plato.  —  Cooling  Curves.     Zs.   Phys.  Ch.,  66,  p.  721;  1906.     68,  p.  350;  1908. 

63,  p.  447. 
Osmond.  —  Cooling  Curves.     C.  R.,  103,  pp.  743,  1112;  1886.     104,  p.  985;  1887. 

Annales  des  Mines,  14,  p.  i;  1888. 
Wiist.  —  Cooling  Curves.     Metallurgie,  3,  p.  i ;  1906. 
Tammann.  — Thermal  analysis.    Zs.  Anorg.  Chem.,  37,  p.  303;  1903.     46,  p.  24, 

1905.     47,  p.  289;  1905. 
Portevin.  —  Thermal  analysis.     Rev.  de  M6tallurgie,  4,  p.  979;  1907.     6,  p.  295; 

1908. 
Dejean.  —  Cooling  Curves.     Rev.  de  Metallurgie,  2,  p.  701;   1905.     3,  p.   149; 

1906. 

Le  Chatelier.  —  Microscopic  Methods.     Rev.  de  Metallurgie,  3,  p.  359;    1906. 
Robin.  —  Hardness  of  Steels  at  High  Temperatures.     Rev.  de  Metallurgie,   6, 

p.  893;  1908. 


488  HIGH  TEMPERATURES 

W.  Rosenhain. — Observations  on  Recalescence  Curves.     Phys.  Soc.  Lond.,  21, 

p.  180;  1908.     Proc.  Inst.  of  Metals. 
G.  K.  Burgess.  —  On  Methods  of  Obtaining  Cooling  Curves.     Electroch.  and  Met. 

Ind.,  6,  pp.  366,  403;  1908.     Bull.  Bureau  Standards, '6,  p.  199;  1908. 
Guertler.  —  Treatise  on  Me"tallographie,  1909. 
Oberhojfer. — Metallographic  Examination  in  Vacuo  at  High  Temperatures.    Metal- 

lurgie,  6,  p.  554;  1909. 
Heyn.  —  Progress  from  1906  to  1909.     Rev.  de  Metallurgie,  7,  p.  34;  1910.     (With 

bibliography.) 

Ruer.  —  Treatise  on  Metallography,  1907. 
Shepherd.  —  Thermometric  Analysis  of  Solid  Phases.     Jl.  Phys.  Chem.,  8,  p.  92; 

1904. 

Desch.  —  Metallography,  1910.     (Longman.) 
Guillet.  —  Traitements  Thermiques,  1909.     (Dunod.) 
Cavalier.  —  Alliages  Me"talliques,  1909. 
Carpenter  and  Keeling.  —  Steels.     Jl.  Iron  and  Steel  Inst.,  p.  224;  1904. 

SPECIFIC    HEAT. 

Iron:     P.  Oberhoffer.     Metallurgie,  4,  pp.  447,  486;  1907.  —  Weiss  and  Beck,  Jl. 

d.  Phys.,  7,  p.  255;  1908.    Harker.  Phil.  Mag.,  10,  p.  430;  1905. —  And  Nickel: 

Lecher.    Phys.  Ges.  Verh.,  9,  p.  647;  1907. 
Carbon:    Kunz.     Ann.  d.  Phys.,  14  p.  309;  1904. 
Gases:    Pier.     Zs.  Elektroch.,  16,  p.  536;  1909.  —  Holborn  and  Austin.     Berlin 

Sitz.  Ber.,  6,  p.   175;  1905. — Holborn  and  Henning.     Ann.  d.  Phys.,  23, 

p.  809;  1907. 
'/ Metals:    Stiicker.     Wien.  Sitz.  Ber.,  114,  p.  657;  1905.  —  Tilden.     Phil.  Trans. 

194,  p.  233;  1900.     201,  p.  37;  1903.     >  A.  fc  K 
Steam:    Holborn  and  Henning.     Ann.  d.  Phys.,  18,  p.  739;  1905. 
Iron-carbon:    Oberhoffer  and  Meuthen.     Metallurgie,  6,  p.  173;  1908.  —  Bystrom. 

Fortschritte  d.  Phys.,  16,  p.  369;  1860. 
Silicates  and  Pt:    White.    Am.  Jl.  Sci.,  28,  p.  334;  1909. 
Platinum:    Plato.    Zs.  Phys.  Ch.,  65,  p.  736;  1906.  —  Violle.     C.  R.,  85,  p.  543; 

1877. 
Copper:    Naceari.    Atti.  di  Torino,  23,  p.  107;  1887.  —  Richards  and  Frazier. 

Chem.  News,  68,  1893.  —  Le  Verrier.     C.  R.,  114,  p.  907;  1892. 
Ferromagnetic  substances:    Dumas.     Arch.  Sci.  Phys.  Nat.,  29,  pp.  352,  458;  1910. 
NH3  and  chemical  equilibrium.     Nernst.    Zs.  Elek.  Ch.,  16,  p.  96;  1910. 


TABLES. 

PAGE 

I.   TEMPERATURE  CONVERSION  TABLE 490 

II.  MELTING  POINTS  OF  THE  CHEMICAL  ELEMENTS 491 

III.  BOILING  POINT  OF  WATER 493 

IV.  BOILING  POINT  OF  SULPHUR 493 

V.  RESISTANCE  THERMOMETER  SCALE  (CENTIGRADE) 493 

VI.   RESISTANCE  THERMOMETER  SCALE  (FAHRENHEIT) 494 

VII.  AUXILIARY  TO  TABLES  V.  AND  VI.. 495 

VIII.  TEMPERATURE  CORRECTIONS  FOR  PLATINUM  OF  DIFFERENT  5 495 

IX.  TRANSFORMATION  TABLE  FOR  ABSORPTION  COEFFICIENTS 496 

X.  ABSORBING  POWERS  FOR  METALS,  ETC 497 


489 


490 


HIGH  TEMPERATURES 


TABLE   I.  — TEMPERATURE  CONVERSION  TABLE. 
(Dr.  L.  Waldo,  in  Metallurgical  and  Chemical  Engineering,  March,  1910.) 


c 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

-200 
—100 
—  0 

-328 
-148 
+  32 

-346 

-166 

+  14 

-364 
-184 
-  4 

-382 
—202 

—  22 

-400 

—  220 

—  40 

-418 
-238 
-  58 

-436 
-256 
-  76 

—454 

-274 
—  94 

-292 

—  112 

-310 

-130 

0 

32 

So 

68 

86 

104 

122 

140 

158 

I76 

194 

C° 

F« 

100 
200 
300 

212 

392 

572 

230 
410 

590 

248 
428 
608 

266 
446 

626 

284 
464 
644 

302 

482 
662 

320 
500 

680 

338 
518 
698 

356 
536 
716 

374 
554 
734 

i 

2 
3 

1.8 
3.6 

5-4 

400 
500 
600 

752 
932 
III2 

770 
950 
1130 

788 
968 
1148 

806 

986 

1166 

824 
1004 

1184 

842 
1022 
I2O2 

860 
1040 

1220 

878 
1058 
1238 

896 
1076 
1256 

914 

1094 
1274 

4 

i 

7-2 
9.0 
10.8 

700 
800 
900 

1292 
1472 
I6S2 

1310 

1490 
1670 

1328 
1508 

1688 

1346 
1526 
1706 

1364 
1544 
1724 

1382 
1562 
1742 

1400 

I58o 
1760 

1418 
1598 
1778 

1436 

1616 
1796 

1454 
1634 
1814 

J 

9 

12.6 

14.4 
16.2 

1000 

1832 

1850 

1868 

1886 

1904 

1922 

1940 

1958 

1976 

1994 

10 

18.0 

1100 

1200 
1300 

2012 

2IQ2 
2372 

2030 

22IO 
2390 

2048 
2228 
2408 

2066 
2246 
2426 

2084 
2264 
2444 

2102 

2282 
2462 

2120 
2300 

2480 

2138 
2318 
2498 

2156 
2336 
2516 

2174 
2354 
2534 

Fo 

C° 

1400 
1500 
1600 

2SS2 
2732 
2912 

2570 
2750 
2930 

2588 
2768 
2948 

2606 

2786 
2966 

2624 
2804 

2984 

2642 
2822 
3002 

2660 
2840 
3020 

2678 
2858 
3038 

2696 
2876 
3056 

2714 

2894 
3074 

I 
2 

3 

0.56 
i.  ii 
1.67 

1700 

1800 
1900 

3092 
3272 
3452 

3IIO 

3290 
3470 

3128 
3308 

3488 

3146 
3326 
3506 

3164 
3344 
3524 

3182 
3362 
3542 

3200 

3380 

3560 

3218 
3398 
3578 

3236 
34i6 

3596 

3254 
3434 
3614 

4 

1 

2.22 
2.78 
3-33 

2000 

3632 

3650 

3668 

3686 

3704 

3722 

3740 

3758 

3776 

3794 

7 

3-89 

2100 

2200 
2300 

3812 

3992 
4172 

3830 
4OIO 
4190 

3848 
4028 
4208 

3866 
4046 
4226 

3884 
4064 
4244 

3902 

4082 
4262 

3920 
4100 

4280 

3938 
4118 
4298 

3956 
4136 
43i6 

3974 

4154 
4334 

8 
9 
10 

4-44 

S.oo 
5.56 

2400 
2500 
2600 

4352 
4532 
4712 

4370 
4550 
4730 

4388 
4568 
4748 

4406 

4586 
4766 

4424 
4604 

4784 

4442 
4622 
4802 

4460 
4640 
4820 

4478 
4658 
4838 

4496 
4676 
4856 

4514 

4694 
4874 

ii 

12 

13 

6.  ii 
6.67 

7.22 

2700 
2800 
2900 

4892 
5072 
5252 

4910 

5090 
5270 

4928 
5108 

5288 

4946 
5126 
5306 

4964 
5144 
5324 

4982 
5l62 
5342 

5000 

5180 
536o 

5018 
5198 
5378 

5036 
5216 

5396 

5054 
5234 
S4I4 

14 
IS 
16 

7-78 
8.33 

8. 

3000 

5432 

5450 

5468 

5486 

5504 

5522 

5540 

5558 

5576 

5594 

17 

9-4* 

3100 
3200 
3300 

5612 

5792 
5972 

5630 
5810 

5990 

5648 
5828 
6008 

5666 
5846 
6026 

5684 
5864 
6044 

5702 

5882 
6062 

5720 
5900 

6080 

5738 
5918 
6098 

5756 
5936 
6116 

5774 
5954 
6i34 

18 

IO. 

3400 
3500 
3600 

6152 
6332 
6512 

6170 
6350 
6530 

6188 
6368 
6548 

6206 

6386 
6566 

6224 
6404 
6584 

6242 
6422 
6602 

6260 

6440 
6620 

6278 
6458 
6638 

6296 
6476 
6656 

6314 

6494 
6674 

3700 
3800 
3900 

6692 
6872 
7052 

6710 

6890 
7070 

6728 
6908 

7088 

6646 
6926 
7106 

6764 
6944 
7124 

6782 
6962 
7142 

6800 

6980 
7160 

6818 
6998 
7178 

6836 
7016 
7196 

6854 
7034 
7214 

C 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

Examples.     1347°  C.  =  2444°  F.+I2.60  F.=24S6.6°  P.;  3367°  F.  =  i8so°  C.+2.780  C.  =  i852.78°  C. 


TABLES 


491 


TABLE  II.  —  MELTING  POINTS  (C.)  OF  THE  CHEMICAL 

ELEMENTS.* 

(Standard  Temperatures  are  in  small  capitals.) 


Element. 

Melting  point. 

Remarks. 

Helium 

<—  269? 

(  B.  P.  He=  -268.5. 

Hydrogen 

—  2<Q 

I  Kamerlingh-Onnes. 
Tra  vers-  Jaci  uerod 

Neon 

*oy 

—  2<J3? 

Oxygen 

**I 

—  230? 

Range  —  227  to  —  235 

Fluorine               

—  223 

Moissan-Dewar 

Nitrogen  

—  2IO.  <? 

Fischer-  Alt. 

Argon  

-188 

Ramsay-  Tra  vers. 

Krypton 

—  160 

Ramsay-Travers 

Xenon 

—  140 

Ramsay-Travers 

Chlorine 

—  101  <? 

Johnson-Mclntosh 

MERCURY         

—  38  7=^0.  <: 

Bromine  
Caesium  

-7-3 

26 

Range  —  7.5  to  —7.0. 
Range  25.3  to  26.5. 

Gallium 

•2Q     I 

Lecoo-Boisbaudran 

Rubidium 

^8 

Range  37.8  to  38.  <?. 

Phosphorus 

44  ! 

Hulett. 

POTASSIUM             

62  3=bo  2 

Sodium  
Iodine  

97,5=1=1.0 
ii4=fc  i 

Sulphur  

113  .  5  to  119.5 

Various  forms. 

Indium 

1^4    S^O    <C 

Lithium 

186 

Kahlbaum. 

Selenium   ...                        .... 

217  to  220 

Various  forms.    Saunders. 

TIN               

27.1    n±o.  2 

Bismuth  

270 

Range  267.5  to  271.5. 

Thallium  

302=*=  I 

CADMIUM 

321    O^O    2 

Range  320.0  to  321.7. 

LEAD 

327.4=fcO.4 

ZINC                           

4IO    4±O.  2, 

Range  418.2  to  419.4. 

Tellurium  

4^1  ^  I 

Arsenic  

(500? 

Guntz-Broniewski. 

ANTIMONY  

(  850 
630=*=  I 

Jolibois. 
'  '  Kahlbaum  '  '  purity  only. 

Cerium 

635 

^lagnesium                          

6<O=fc2 

ALUMINIUM                   

658=*=! 

Calcium           

805  ±5 

Lanthanum 

810? 

Muthmann-Weiss. 

Strontium                                  .  . 

>Ca,<Ba? 

Neodymium                          .... 

840? 

Muthmann-  Weiss. 

Barium                           

850 

Guntz. 

Germanium     

<Ag 

Winkler. 

Praseodymium 

? 
940  r 

Muthmann-Weiss. 

SILVER                                  .... 

96l=fc  2 

Radium                               

6OO  tO  I2OO? 

Unknown. 

GOLD                          

1063=*=  3 

*  G.  K.  Burgess,  Jl.  Wash.  Acad.  Sci,,  i,  p.  16, 1911. 


4Q2  HIGH  TEMPERATURES 

MELTING  POINTS  (C.)  OF  THE  CHEMICAL  ELEMENTS.    (Cont'd.) 


Element. 

Melting  point. 

Remarks. 

COPPER 

1081=*=  3 

Manganese 

122^=*=  IS 

Yttrium 

1000  to  1400? 

Unknown. 

Samarium  ....       

1300  to  1400 

Muthmann-  Weiss. 

Scandium  

1000  to  1400? 

Unknown. 

Silicon  

1420=*=  15 

NICKEL  

1450=*=  10 

Day-Sosman  =  1452. 

Cobalt 

I4QO 

Day-Sosman. 

Chromium 

IZOZ^IZ 

IRON 

1^20=*=  IS 

PALLADIUM  

IS1?©1*1  I1? 

Day-Sosman  =  1549. 

Zirconium  

>  Silicon 

Troost. 

Thorium  

>i7oo,  <Pt 

Wartenberg. 

Vanadium 

17-20=*=  to 

PLATINUM  

1755=*=  20 

Waidner-Burgess  =  1  753  . 

Beryllium 

>i8oo 

Parsons. 

Ytterbium 

1600  to  2OOO? 

Unknown. 

Titanium     

(  22OO  tO  24OO? 

Weiss-Kaiser. 

Rhodium  

\  1800  to  1850 

IQ2O? 

Hunter. 
Range  1907  to  1970. 

Ruthenium  

>i95o 

Joly. 

Niobium  

2  2OO? 

v.  Bolton=i95o. 

Boron  

2200  to  2<;oo 

Weintraub. 

Iridium 

_° 
2300? 

Range  2100  to  2350 

Uranium 

near  Mo 

Moissan. 

Molybdenum 

2500? 

Range  2110  to  >25oo. 

Osmium   .           .... 

2700? 

Waidner-Burgess. 

Tantalum  

2850 

Waidner-Burgess  =  2910. 

TUNGSTEN  

3006  =±=100 

(  Range  2575  to  3250. 

Carbon  

? 

I  Waidner-Burgess  =  3080. 
Unknown. 

TABLES 


493 


TABLE   III.  — BOILING  POINT  OF  WATER. 

Temperature  Centigrade;   Barometer  in  mm.  of  Mercury. 


mm. 

0 

I 

2 

3 

4 

5 

6 

7 

8 

9 

730 

98.880 

98.918 

98.956 

98.994 

99.032 

99  069 

99-107 

99-145 

99-183 

99.220 

740 

99-258 

99.295 

99-333 

99-370 

99.407 

99-445 

99.482 

99  519 

99-557 

99-594 

750 

99-631 

99-668 

99.705 

99-742 

99-779 

99.816 

99.853 

99.890 

99.926 

99.963 

760 

100.  OOO 

100.037 

100.073 

100.  IIO 

100.146 

100.183 

100.219 

100.256 

100.292 

100.327 

TABLE   IV.  — BOILING  POINT  OF  SULPHUR. 

Temperature  Centigrade;    Barometer  in  mm.  of  Mercury. 


mm. 

o 

I 

2 

3 

4 

5 

6 

7 

8 

9 

730 

442.00 

442.09 

442.18 

442.27 

442.36 

442.45 

442.53 

442.62 

442.71 

442.80 

740 

750 

442.89 
443-79 

442.98 
443.88 

443.07 
443-97 

443.16 
444.06 

443-25 
444-15 

443-34 
444-24 

443-43 

444.34 

443-52 

444-43 

443.61 
444  52 

443.70 
444.61 

760 

444-70 

444-79 

444-88 

444-97 

445.06 

445-15 

445-25 

445-34 

445-43 

445-52 

I 

This  table  is  based  on  the  assumption  that  the  normal  boiling  point  of  sulphur 
is  "444.70.  The  other  temperatures  are  computed  by  Holborn  and  Henning's 
formula. 

TABLE  V.  — RESISTANCE  THERMOMETER  SCALE 

(CENTIGRADE). 

Values  of  Temperature  Centigrade  (t)  in  Terms  of  Platinum  Temperatures 
(pt)  for  Thermometers  with  d  =  1.500. 


pt 

t 

Differ- 
ence for 
i°  PL 

Pt 

/ 

Differ- 
ence for 
1°  pt. 

Pt 

* 

Differ- 
ence for 
i°  pt. 

Pt 

t 

Differ- 
ence for 
i°  pt. 

0 

o.ooo 

.985 

250 

255.99 

.066 

500 

534-89 

.170 

750 

844.26 

313 

IO 

9.867 

.988 

260 

266.67 

.070 

Sio 

546.62 

-175 

760 

857.42 

•  319 

20 

19.762 

.991 

270 

277-38 

.073 

520 

558.40 

.180 

770 

870.65 

.326 

30 

29.687 

.994 

280 

288.13 

.077 

530 

570.22 

.185 

780 

883.95 

•  333 

40 

39.641 

.997 

290 

298.92 

.081 

540 

582.10 

.190 

790 

897.32 

340 

So 

49.625 

.000 

300 

309.75 

.084 

550 

594-03 

•  195 

800 

910.76 

.  -347 

60 

59.639 

.003 

3io 

320.61 

.088 

56o 

606.00 

.200 

810 

924.28 

•  355 

70 

69.683 

.006 

320 

33i  5i 

.092 

570 

618.03 

.205 

820 

937.87 

.363 

80 

79.758 

.009 

330 

342.46 

.096 

58o 

630.11 

.210 

830 

951-54 

•  370 

90 

89.863 

.012 

340 

353-44 

.100 

590 

642.24 

.216 

840 

965-28 

.378 

100 

IOO.OO 

.015 

350 

364.46 

.104 

000 

654.43 

.222 

850 

979-10 

.386 

IIO 

110.17 

.018 

360 

375-52 

.108 

610 

666.67 

.227 

860 

993  oi 

•  394 

120 

120.37 

.021 

370 

386.62 

.112 

620 

678.97 

.232 

870 

1007.00 

.403 

130 

130.60 

.024 

38o 

397-76 

.116 

630 

691.32 

.238 

880 

1021.07 

.4" 

140 

140.86 

.027 

390 

408.95 

.120 

640 

703-73 

•  244 

890 

1035-23 

.420 

150 

151.16 

.031 

400 

420.18 

-125 

650 

716.20 

.250 

900 

1049.47 

.428 

160 

161.49 

.034 

410 

431-45 

.129 

660 

728.73 

.256 

910 

1063.80 

•  437 

170 

171.85 

.038 

420 

442-77 

.134 

670 

741.32 

.261 

920 

1078.21 

•  445 

180 

182.25 

.041 

430 

454-13 

.138 

680 

753-97 

.267 

930 

1092.71 

.455 

190 

192.68 

-044 

440 

465.53 

.142 

690 

766.67 

.274 

940 

1107.31 

.464 

200 

203.14 

.048 

450 

476.97 

.146 

700 

779-44 

.280 

950 

1122.00 

•  474 

2IO 

213.64 

.052 

460 

488.46 

.151 

710 

792.27 

.286 

960 

1136.79 

.484 

220 

224.18 

055 

470 

500.00 

.156 

720 

805.17 

•  293 

970 

1151.69 

•  494 

230 

234.75 

.058 

480 

5ii-58 

.160 

730 

818.13 

•  299 

980 

1166.68 

•  503 

240 

245.35 

.062 

490 

523.21 

.165 

740 

831.16 

.306 

990 

Il8l.76 

•  513 

250 

255.99 

.066 

500 

534.89 

.170 

750 

844-26 

.313 

1000 

1196.95 

.524 

494 


HIGH  TEMPERATURES 


TABLE  VI.  — RESISTANCE  THERMOMETER  SCALE  (FAHR.). 

5=1.50. 


Platinum 

Gas  scale 

Platinum 

Gas  scale 

Platinum 

Gas  scale 

Platinum 

Gas  scale 

tempera- 

tempera- 

tempera- 

tempera- 

tempera- 

tempera- 

tempera- 

tempera- 

tures. 

tures. 

tures. 

tures. 

tures. 

tures. 

tures. 

tures. 

o 

0.56 

510 

522.7 

1040 

II22.8 

1570 

1805.5 

10 

10-35 

520 

533-4 

1050 

1134.8 

1580 

1819.4 

20 

20.19 

530 

544-2 

1060 

1146.9 

1590 

1833.4 

30 

30.03 

540 

554-9 

1070 

H58.9 

l6oo 

1847.4 

32 

32.0 

550 

565-7 

1080 

II7I.O 

1610 

1861.6 

40 

39-9 

560 

576.5 

1090 

1183.1 

1620 

1875-6 

5° 

49-8 

570 

587-3 

IIOO 

"95  3 

1630 

1889.9 

60 

59-7 

580 

598.2 

i  no 

1207.5 

1640 

1904.1 

70 

69-5 

590 

609.1 

1  1  20 

1219.7 

1650 

1918.3 

80 

79-5 

600 

620.0 

1130 

1232.0 

1660 

1932.5 

QO 

89.4 

610 

630.9 

1140 

1244-3 

1670 

1946  .  8 

100 

99  4 

620 

641.8 

1150 

1256.6 

1680 

1961  .  2 

no 

109.3 

630 

652.8 

1160 

1270.0 

1690 

1975-7 

1  20 

"9-3 

640 

663.8 

1170 

1281.3 

1700 

1990.2 

130 

129.3 

650 

674.8 

1180 

1293.7 

1710 

2004.7 

140 

139-4 

660 

685.8 

1190 

1306  .  i 

1720 

2019.3 

150 

149.4 

670 

696.9 

1200 

1318.7 

1730 

2034.0 

160 

159-4 

680 

707-9 

I2IO 

1331-1 

1740 

2048  .  7 

170 

169.5 

690 

719.0 

I22O 

1343  •  7 

1750 

2063.4 

180 

179.6 

700 

730.1 

1230 

1356-3 

1760 

2078.2 

190 

189.7 

710 

741-3 

I24O 

1368.9 

1770 

2093.1 

200 

1998 

720 

752.5 

1250 

1381.5 

1780 

2108.0 

210 

209.9 

730 

763-6 

1260 

1394-2 

1790 

2123.0 

212 

212.  0 

740 

774-8 

1270 

1406  .  9 

1800 

2138.0 

22O 

22O.I 

750 

786.0 

1280 

1419.6 

1810 

2I53-I 

230 

230.3 

760 

797-3 

1290 

1432.4 

1820 

2168.3 

240 

240.5 

770 

808.6 

1300 

1445   2 

1830 

2183.5 

250 

250.7 

780 

819.9 

1310 

I458.I 

1840 

2198.7 

260 

260.9 

790 

831.2 

1320 

I47I.O 

1850 

2213.0 

270 

271.2 

800 

842.6 

1330 

1483.9 

1860 

2229.4 

280 

281.4 

810 

854.0 

1340 

1496.8 

1870 

2244-9 

290 

291.7 

820 

865.4 

1350 

1509.8 

1880 

2260.4 

300 

302.0 

830 

876.8 

1360 

1522.9 

1890 

2276.0 

310 

312.3 

840 

888.3 

1370 

1535-9 

1900 

2291.6 

320 

322.7 

850 

899.7 

1380 

I549-I 

1910 

2307-3 

330 

333-0 

860 

911.2 

1390 

1562.1 

1920 

2323.0 

340 

343-4 

870 

922.7 

1400 

1575  3 

1930 

2338.9 

350 

353-8 

880 

934-3 

I4IO 

1588.5 

1940 

2354.8 

360 

364.2 

890 

945-9 

I42O 

1601.8 

1950 

2370.8 

370 

374-6 

900 

957  5 

1430 

1615.1 

1960 

2386.8 

380 

385-1 

910 

969.1 

1440 

1628.4 

1970 

2402  .  9 

390 

395-6 

920 

980.8 

1450 

1641.8 

1980 

2419.0 

400 

406.1 

930 

992.5 

1460 

1655.2 

1990 

2435-3 

410 

416.6 

940 

1004.2 

1470 

1668.7 

2000 

2451.6 

420 

427.1 

950 

1015.9 

1480 

1682.2 

2010 

2468.0 

430 

437-6 

960 

1027.7 

1490 

1695-7 

2O2O 

2484.4 

440 

448.2 

970 

1039-5 

1500 

1709  3 

2030 

2500.8 

450 

458.8 

980 

1051-3 

1510 

1722.9 

2O4O 

2517-5 

460 

469.4 

990 

1063  .  i 

1520 

1736.6 

2050 

2534-2 

470 

480.0 

1000 

1075.0 

1530 

1750.3 

2O6O 

2551.0 

480 

490.6 

IOIO 

1086.9 

I54O 

1764.0 

49O 

501  .3 

IO2O 

1098  .  8 

I55O 

1777.8 

500 

512.0 

1030 

ino.8 

1560 

1791.6 

TABLES 


495 


TABLE  VII.  — AUXILIARY  TO  TABLES  V  AND  VI. 

Corrections  to  t  for  small  changes  in  S. 


Centigrade  scale. 

Fahrenheit  scale. 

At  tor 

A/  for 

Affor 

At  for 

A5  =  o.oi. 

A5  =  o.oi. 

AS  =  o.oi. 

A6  =  o.oi. 

5° 

—  O.OO2 

550 

+0.247 

IOO 

—  0.003 

IIOO 

+0-53 

100 

.OOO 

600 

.300 

200 

.OOO 

1  200 

.64 

150 

+    .008 

650 

•357 

300 

+    .014 

1300 

.76 

2OO 

.O2O 

700 

.420 

4OO 

.038 

1400 

.90 

250 

•037 

750 

.487 

500 

.07 

1500 

•OS 

300 

.060 

800 

.560 

600 

.11 

1600 

•23 

350 

.087 

850 

•637 

700 

.18 

1700 

•38 

400 

.I2O 

QOO 

.720 

800 

•25 

1800 

•56 

450 

•157 

950 

.807 

900 

•33 

1900 

•73 

500 

.  2OO 

1000 

.900 

IOOO 

.42 

2000 

•95 

Computations  of  /  from  pt  are  made  by  Table  V,  as  if  the  thermometer  had  d  = 
1.50.  The  above  corrections  (A/)  are  then  applied  to  the  computed  values  of  t 
for  the  value  of  5  proper  to  the  thermometer. 

Example.  Let  pt  =  470.00,  whence  /  =  500.00°  C.  by  Table  V.  If  5  =  1.52, 
the  corrected  value  of  /  is  500.40°  C.  by  Table  VII. 


TABLE  VIII.  —  TEMPERATURE  CORRECTIONS  FOR  PLATINUM 
OF  DIFFERENT  5. 

[Thermometer  calibrated  by  Callendar  method,  ice,  steam,  and  S.  B.  P.J 


Correction  in  °  C.  for  values  of  S  given  below. 


°c. 

1.525 

1.550 

1.575 

i.  600 

1.650 

1.700 

1.800 

1.900 

200 

+O.O2 

+0.05 

+   0.08 

+  o.  10 

+    0.14 

+  0.16 

+    O.2O 

+    O.2I 

300 

+     .02 

+    -05 

+       .08 

+     .11 

+       -19 

+     -27 

+     -45 

+     -55 

400 

.OO 

.00 

+       .01 

+     -03 

+       .08 

+      -14 

+     .29 

+     -37 

500 

—     .02 

—    -05 

-       .09 

—     .  ii 

-       .18 

-      -24 

-      -39 

-      -57 

600 

-     .09 

-    .18 

-       -30 

-     .40 

-        .62 

-     .88 

-   1.42 

-   1.96 

700 

-    -33 

-    .70 

-    1.03 

-   1.32 

-    I.78 

—    2.2 

-  2.9 

-  3-5 

800 

-  .90 

-1.65 

-2.24 

—  2.7 

~    3-6 

-    4.4 

-  5-8 

-   7.1 

900 

—1.90 

-3-i 

—    4.0 

—  4-9 

-    6.5 

-  8.1 

—  10.8 

-13-5 

IOOO 

-3-3 

-5-2 

-  6.8 

-  8.2 

-10.7 

-13.1 

—  17.1 

-20.8 

IIOO 

-5-5 

-8.1 

-10.3 

—  12.2 

-15-7 

-18.7 

-24-3 

-29.1 

The  above  table  applies  only  when  the  value  of  5  is  that  given  by  using  the 
S.  B.  P.  as  third  calibration  point  of  a  resistance  thermometer. 


496 


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TABLES 


497 


TABLE  X. 


ABSORBING  POWERS*  FOR  POLISHED  METALS 
AND  OTHER  SUBSTANCES. 


^^ValuesofX. 

Blue, 

Green, 

Orange, 

Red, 

I 

nfra  red. 

Substance.  ^\^^ 

0.4 

o.S 

0.6 

0.7 

2.O 

5-0 

8.0 

Silver                     

o.  16 

O.  IO 

O.O75 

0.058 

O.O2I 

O.OI^ 

O.OI2 

Gold                     

.72 

•53 

.156 

.077 

.032 

.030 

.O2O 

Platinum  

.52 

.42 

.36 

.31 

.19 

.07 

.05 

Palladium 

42 

.36 

.31 

Rhodium 

•  24 

•  2^ 

.21 

.OO 

.07 

.06 

Iridium.                         .  .  . 

•  25 

•  25 

•  24 

.14 

.06 

.05 

Iron                           

.50 

•45 

.42 

.41 

.22 

.09 

.06 

Copper: 
Liquid  

•35 

.  20 

.15 

Solid 

•47 

•  *7 

.OQ 

Nickel 

•47 

•  30 

•  35 

.31 

.17 

.06 

.05 

Tungsten  
Tantalum              

•53 

•Si 
.62 

•49 
•55 

•46 

•45 

•15 
.IO 

.06 
.07 

.04 
.06 

Molybdenum  

.56 

•55 

.52 

•5° 

.18 

.08 

.06 

Chromium  

•45 

•45 

•44 

•37 

.19 

.11 

Vanadium 

•44 

.4? 

.42 

.32 

.18 

.IO 

Antimony 

•47 

.40 

Magnesium 

.28 

.27 

.23 

.14 

.07 

Tellurium  
Silicon        

•52 
.66 

•52 
.68 

•52 
•7° 

•49 

•72 

•41 

.72 

.28 

.72 

Graphite: 
Polished 

70 

78 

•  77 

.76 

.65 

•5° 

•45 

Matt 

>.QO 

>.QO 

Cuprous  oxide 

•7° 

•7° 

•  65 

.60 

Iron  oxide 

65     t 

O      .QO 

>  oo 

>  oo 

>  oo 

2"?       t 

o      so 

07      t 

o    .  13 

IO 

IO 

06       t 

O        OQ 

.06       t 

o    .09 

Lime 

.10      t 

o    .40 

•  The  absorbing  power  a  =  emissive  power  e  =  i  -  r,  where  r  =  reflecting  power. 


INDEX 


ABBOT,  268,  274,  276,  277,  455. 
Absorption  Coefficient,  254,  300,  335, 
336. 

pyrometer,  311. 
ACHESON,  1 80. 
Actinometer,  263. 
Air,  as  thermometric  gas,  18,  19,  23,  25, 

3i,  35,  61. 

Alarm,  temperature,  431. 
ALLEN,  452. 
Alloys  (see  also  thermocouples). 

melting  points  of,  451. 

chemical  changes  in,  107. 
Aluminium,  freezing  or  melting,  445, 

449- 

in  thermocouples,  170. 
AMAGAT,  34. 
ANGSTROM,  276. 
Aniline,  boiling  point,  450. 
Antimony,  melting  and  freezing  point, 

62,  445,  447. 

Arc,  carbon,  temperature  of,  273,  453. 
Argon,  as  thermometric  gas,  26. 
ARNOLD,  191. 
ARSEM,  333,  342,  461. 
ARSONVAL  (D'),  n,  121,  135,  137,  212, 

267,  271. 
AUSTIN,  76,  159. 
AVENARIUS,  in. 
AYRTON,  122. 

BARR,  443. 
BARRETT,  169. 

BARUS,  9,  n,  58,  76,  77,  103,  171,  378. 
boiling  and  freezing  points,  6,  187, 

188,  438,  443- 
gas  thermometer,  65. 
thermoelectric  pyrometer,  102,  105, 
112,  118. 


BAUER,  247,  340. 
BECK,  specific  heats,  92,  93. 
BECKER,  340. 
BECKMANN,  251. 

BECQUEREL,  iii,  iv,  9,  10,  n,  15  16,  51, 
54,  57,  63,  109,  296. 

gas  thermometer  of,  62. 

melting  and  boiling  points,  438,  439, 
441. 

thermoelectric  pyrometer,  101,  121. 
BEDFORD,  59. 

BELLOC,  thermoelectricity,  no,  169. 
BENEDICKS,  421. 
BENOIT,  55. 

Benzophenone,  boiling  point,  450. 
BERTHELOT,  99. 

calorimetry,  94. 
BERTHELOT,  DANIEL,  180,  188. 

expansion  of  gases,  25. 

gas  thermometer  of,  85. 

melting  and  boiling  points,  8,  88,  438, 
439,  440,  441,  445- 

thermodynamic  corrections,  31,  32, 

34,  35- 

Bibliography,  465. 
Biju-DuvAL,  calorimetry,  97. 
Bismuth,  freezing  point,  445. 
Black  body,  239,  247,  293,  333. 

temperature,  242. 
Boiling  points,  6,  88,  226. 

methods  for,  187. 

standard,  463. 

Bolometer,  248,  268,  274,  425. 
v.  BOLTON,  446. 

BOLTZMANN,  248,  251. 

radiation  laws,  247. 

BOTTOMLY,  247. 

BOUDOUARD,  v,  vii,  305,  420. 
BOYLE,  Law  of,  13. 


499 


500 


INDEX 


BOYS,  10,  267,  272. 

BREAJRLEY,  366,  413. 

Br.  Association  and  platinum  thermom- 

etry,  194,  227. 
BREGUET,  158. 
BRISTOL,  83,  156,  157,  170,  175,  408, 

411. 

BROCA,  213. 
BROWN,  284,  285. 
BUCKINGHAM,  corrections  to  gas  scales, 

32,  34,  35- 

BUDENBERG,  380. 
BUNSEN,  276. 

Bureau    International    des    Poids    et 

Mesures,  13,  20,  22,  24. 
gas  thermometer  of,  37. 
BURGESS,  iii,  v,  vi,  viii,  10,  79,  252,  330, 

334,  414,  416. 

freezing  and  boiling  points,  7,  199, 
226,  435,  437,  438,  441,  442,  443, 
444,  445,  451,  452,  454,  459. 

melting  points,  9,  344,  446. 

optical  and  radiation  pyrometry,  240, 
243,  253,  286,  307,  320,  323,  332, 

335,  337,  340,  344- 

resistance  pyrometer,  199,  200,  205, 

228,  231,  232,  233. 
thermoelectric  pyrometer,  115,  185. 
BURTON,  377. 

Cadmium,  boiling  point,  66,  88. 

freezing  point,  445. 
CAGNIARD-LATOUR,  380. 
CALLENDAR,  9,  10,  12,  59,  68,  74,  198, 
199,  200,  211,  213,  225,  231,  234, 
269,  274,  276,  378. 
gas  expansion,  35. 
gas  scale  corrections,  21,  31,  32. 
gas  thermometer  of,  70. 
melting  and  boiling  points,  6,  226, 

434,  435,  438. 
recording  pyrometers,  386,  388,  390, 

425- 
resistance  pyrometer,  194,  195,  196, 

197,  201,  202,  217. 
Calorimeters,  94. 
Calorimetric  pyrometer,  10,  89. 


Calorimetric  pyrometer,  precision  of,  97, 

use  of,  99. 
CAMBRIDGE    SCIENTIFIC    INSTRUMENT 

Co.,  131,  143,  176,  194,  203,  213, 

223,  237,  390,  408,  409,  413,  425,. 

426. 
Carbonic  acid,  as  thermometric  gas,  i8r 

19,  20,  22,  25. 
Carbon    monoxide,    as    thermometric 

gas,  1 8. 

CARHART-CLARK,  136. 
CARNELLY,  377,  378. 
CARNOT,  26. 
Carnot's  principle,  26. 
CARPENTER,  416. 
CARPENTIER,  124,  125,  145,  217,  221,, 

393,  408. 

Cell,  standard,  136. 
CELSIUS,  scale  of,  3. 
CHAPPUIS,  9,  21,  24,  30,  31,  34,  54,  59, 

68,  70,  72,  198. 
expansion  of  gases,  21,  25,  35. 
gas  scales,  20,  23,  434. 
standard  gas  thermometer,  42. 
standard  mercury  thermometers,  44, 
CHARPY,  191,  403,  404. 
CHAUVIN  and  ARNOUX,  132,  176,  408. 
Chromium,  in  thermocouples,  106,  170. 
CLAPEYRON,  380. 
CLARK,  136,  137,  428. 
Clays,  heating  curves,  399. 
CLEMENT,  68,  76,  79. 
gas  thermometer,  75. 
melting  points,  8,  440. 
Cobalt  glass,  347. 
Cobalt,  melting  point,  446. 
Color  scale,  346. 

COBLENTZ,  253,  268,  269,  341,  342. 

Cones  (fusible  or  Seger),  n,  368. 

scale  of,  371,  373. 
Constantan,  167,  174. 
Contraction  pyrometer,  n,  357. 
Cooling  curves,  383. 

derived  differential,  384. 

differential,  382,  414. 

inverse  rate,  384. 

methods  for,  384,  396,  408. 


INDEX 


Cooling  curves,  rapid,  420. 

temperature-rate,  382. 

time- temperature,  381. 

with  neutral,  414. 
Copper,  emissivity,  286. 

eutectics,  441. 

in  thermocouples,  167,  169. 

melting  or  freezing  point,  65,  441, 

447- 

specific  heat,  93. 
CORNU,  296,  298. 
COUPEAUX,  58. 
CRAFTS,  57,  81,  84,  85,  380. 

boiling  points,  6,  434. 

expansion  of  gases,  22. 

gas  thermometry,  49,  51,  83. 
CRAMER,  369. 
CROMPTON,  159. 
CROOKES,  267. 
CROVA,  350,  352. 
Crucibles,  181,  188. 

furnaces,  180,  459. 
Curve  tracer,  412. 
Curves,  freezing  point,  447. 


DARLING,  158. 
DARWIN,  431. 

DAY,  vii,  viii,  9,  54,  59,  67,  68,  72,  76, 
77,  79,  81,  114,  iSS,  160,  161,  171, 
180,  253,  440,  456. 
expansion  of  metals,  55. 
gas  thermometer,  75. 
melting  points,  7,  8,  55,  438,  439,  441, 

442,  445,  446,  451,  452. 
thermoelectric  pyrometer,  112,  113. 
DEJEAN,  recording  pyrometer,  382,  399, 

400. 

DEPREZ,  n,  121,  137,  271. 
DICKINSON,  99,  206. 
DICKSON,  resistance  pyrometer,  200. 

DlESSELHORST,  145,  146. 

Displacement,  Wien's  law  of,  251. 
DIXON,  1 80. 
DOERNICKEL,  171. 

DULONG,  245,  264,  265,  266. 

DUMAS,  85. 


EDELMANN,  421. 

EDWARDS,  230. 

EINTHOVEN,  421. 

Electromotive  forces  of  thermocouples, 

105,  in,  116. 

Electrothermal  recorder,  394. 
Emissive  powers,  239,  244,   255,   291, 

293- 

correction  for,  255,  257. 

determination  of,  301. 
Energy  curves,  248. 
EUCHENE,  specific  heats,  92,  93. 
EUMORFOPOULOS,  59,  68,  74,  75,  200, 

367,  434- 
Eutectics,  451,  452. 

Cu  .  Cu2O,  441,  445- 

Ag8.Cu2,  445,  451- 


FAHRENHEIT,  scale  of,  3. 

FARYTHER,  333. 

FERY,  vii,  10,  247,  268,  283,  285,  286,. 

333,  34i,  443,  454,  455- 

radiation      pyrometers,      277,     280, 
281,     284,    311,    314,    339,    354,. 
425- 
Filaments,  temperature  of  lamp,  340. 

temperature  of  Nernst,  342. 

wide  comparison,  331. 
Fixed  points,  5,  433. 

table  of,  456. 
FISCHER,  155. 
FIZEAU,  43,  55,  61. 
Flames,  temperature  of,  338. 
Flicker  photometer,  352. 
Fluorite,  278. 
FOSTER,  284,  285. 
FOURNIER,  12,  380. 

FOWLE,  276,  455. 

FRAZIER,  specific  heats,  94. 

Freezing  points  (see also  Melting  points). 

curves  of,  447. 

reproducibility,  452. 
Furnaces,  179,  289,  458. 

control  of,  430. 

temperatures,  191,  388. 
Fusible  cones,  n,  368. 


502 


INDEX 


Galvanometer,  for  radiation  pyrometers, 

283.  ^ 
for  resistance  pyrometers,  209,  212, 

213,  216,  220. 
for    thermocouples,    118,    120,    131, 

J33,  137. 
string,  421. 
Gases,  coefficients  of  expansion,  17,  25, 

35- 

variations  of,  18,  20,  22. 

at  high  temperatures,  26. 

critical  constants,  32. 

scale  corrections  for,  31. 
GASPARIN,  262. 

Gas  pyrometer  (see  also  Gas  thermom- 
eter), 9,  37. 
•Gas  thermometer,  3,  14,  16. 

as  standard,  13,  16,  37,  430. 

comparison  of  results  with,  79. 

constant  pressure,  15,  50,  64,  70. 

constant  volume,  14,  44,  64,  67,  68, 

75,  76. 

formulae  and  corrections,  44,  74. 

future  experiments  with,  80. 

indirect  processes,  83. 

industrial,  82. 

method  of  D.  Berthelot,  85. 

method  of  Crafts  and  Meier,  83. 

methods  of  Regnault,  84. 

methods  of  Sainte-Claire-Deville,  85. 

of  variable  pressure  and  mass,  15. 

recording,  385. 

substance  of  bulbs,  53. 

volumetric  thermometer,  15,  51. 
GAY-LUSSAC,  law  of,  13,  27. 
GEIBEL,  thermoelectric  pyrometer,  171. 
Geophysical  laboratory,  vii,  8,  55,  56, 

77,  181,  342,  460. 
Glass,  absorbing,  300. 

as  gas  thermometer  bulbs,  59,  68,  71. 

cobalt,  347. 

colored   (monochromatic),   297,  334, 
347- 

thermometric,  17,  363. 
Gold,  in  thermocouples,  172. 

melting  or  freezing  point,  62,  65,  67, 
88,  272,  439,  445. 


GOLDHAMMER,  456. 

Graphite,  furnaces,  462. 
GRAU,  341. 

GRAY,  271,  272,  273,  305,  454. 
GREENWOOD,  342,  463. 

boiling  points  of  metals,  9. 
GRENET,  366. 
GRIFFITHS,  74,  212,  214. 

melting  and  boiling  points,  6,  226, 

433,  434- 

resistance  pyrometer,  194,  195,  197. 
GURNEISEN,  59. 
GUILLAUME,  244,  364. 

GWYER,  226. 

HADFIELD,  v,  169. 

BARKER,  21,  24,  30,  68,  72,  437. 

melting  and  boiling  points,  434,  443. 

resistance  pyrometer,  198. 

specific  heats,  92. 

thermoelectric  pyrometer,  114,  143. 
HARRIS,  218. 
HARRISON,  169. 
HARTMANN,  323,  342. 
HARTMANN  and  BRAUN,  127,  158,  218, 

408,  409. 
HAUSRATH,  146. 
Heat  (see  also  Specific  heat). 

total,  of  metals,  90. 
HECHT,  369. 

HENNING,  60,  68,  79, 159,  228,  256,  331, 
336,  437- 

melting  and  boiling  points,  8,   226, 

434,  435- 

optical  pyrometer,  330. 
HER^US,  60,  75, 152, 164,  171, 174,  180, 

181,  205,  226,  331,  376,  458,  459. 
HEVESY,  174. 
HEYCOCK,  440,  452. 

melting  points,  6,  199,  438,  439,  440, 

44i,  443- 

resistance  pyrometer,  194,  199,  202, 
231. 

HOBSON,  12. 

HOFFMANN,  376,  418. 
HOLBORN,  vii,  9,  10,  54,  56,  57,  59,  60, 
68,  72,  76,  79,  159,  161,  171,  180, 


INDEX 


503 


200,  228,  247,  253,  328,  330,  337, 
340,  342,  343,  345,  437,  44Q. 
boiling  points,  8,  226,  434,  435. 
expansion  of  metals,  55. 
gas  pyrometer,  67,  75. 
melting  points,  7,  8,  67,  438,  439,  44i, 

442,  443,  444,  445. 
optical  pyrometer,  324,  325,  327,  329, 

426. 
resistance  pyrometer,  196,  199,  233, 

234- 
thermoelectric  pyrometer,  112,  113, 

119,  126. 
HOLMAN,  378,  442,  443. 

thermoelectric  pyrometer,  112,  113, 

114,  139. 

HOSKINS,  170,  411. 
HOVESTADT,  364. 
HOWE,  191. 

HULETT,  137. 
HiJTTNER,  367. 
HUTTON,  462. 

Hydrogen,  as  thermometric  gas,  18,  19, 

20,  21,  22,  25,  31,  35,  68. 
Helium,  as  thermometric  gas,  26,  31, 35. 


Ice  point,  34. 
INGERSOLL,  342,  444,  445. 
International  committee  on  weights  and 

measures,  scale  of,  37. 
Iodine  vapor,  in  gas  thermometry,  62, 

84. 
Iridium,  expansion  of,  56. 

furnace,  459. 

gas  thermometer  bulbs,  56,  75. 

in  resistance  pyrometers,  235. 

in  thermocouples,  172,  174. 

melting  point,  294,  444. 

specific  heat,  91. 
Iron,  emissivity,  256,  287. 

expansion  of,  56. 

gas  thermometer  bulbs,  56,  62. 

in  thermocouples,  101,  103,  no,  168. 

melting  point,  446. 

specific  heat,  92. 
Isochromatic  curves,  250. 


JAQUEROD,  60,  61,  68. 

boiling  and  melting  points,  7,  437, 
439,  440. 

expansion  of  gases,  25. 

gas  thermometer,  70. 
JAGER,  211,  231,  451. 
JOB,  12,  378. 
JOLY,  n,  220,  272,  341. 

meldometer,  361. 
JOULE,  21,  27,  30,  31,  34. 

KAHLBAUM,  453. 

KANOLT,  342,  375,  376. 

KEISER  and  SCHMIDT,  127. 

KELVIN  (THOMSON),  21,  30,  31,  34,  109, 

215,  274. 

Kelvin  bridge,  with  resistance  pyrom- 
eter, 215. 
KING,  247. 
KIRCHHOFF,  240,  244,  245,  333. 

radiation  laws,  239,  243. 
KOLOWRAT,  430. 

KONIG,  315. 

KURLBAUM,  Vii,  10,  247,  328,  330,  337, 

342,  343,  346,  455- 
optical  pyrometer,  324,  325,  327,  329, 

426. 
radiation  laws,  238,   240,   246,  274, 

338,  340. 

KURNAKOW,  403,  404. 

Laboratorie  d'Essais,  457. 

LADENBURG,  340. 

Lamps,  pyrometer,  328,  329,  332. 

comparison,  332. 

electric,  temperature  of,  340. 
LANGLEY,  10,  248,  268,  274. 

bolometer,  267,  273,  425. 
LATIMER-CLARK,  136. 
LAUTH,  368. 
LAWRENCE,  443. 
Lead,  freezing  point,  445. 
LE  CHATELIER,  vi,  vii,  10,  n,  12,  51, 65, 
67,  97,   103,   104,   171,   172,   182, 
278,  306,  3",  3U,  3i8,  333,  336> 
340,  346,  369,  397,  421,  454. 

contraction  pyrometer,  358. 


504 


INDEX 


LE  CH ATELIER,  expansion  of  porcelain, 

58. 
optical  pyrometry,  292,  296,  301,  302, 

305- 
recording  pyrometers,  382,  396,  400, 

418,  420. 
specific  heats,  93. 

thermoelectric  pyrometer,  102,  105, 
107,  108,  in,  119,  122,  125,  128,. 
191. 

LEEDS  and  NORTHRUP,  140,  145,  203, 
205,  208,  209,  212,  216,  218,  224, 
235,  236,  393,  408,  420. 
LEHRFELDT,  21. 
ice  point,  34. 
LEITHAUSER,  253. 
LESLIE,  266. 

LE  VERRIER,  specific  heats,  94. 
Logometer,  221. 
LOUGININE,  99. 
Luminous  intensity,  293. 
LUMMER,  vii,  338,  340,  454- 
radiation  laws,  238,   239,  240,  246, 
248,  250,  251,  252,  274,  352. 

McCRAE,  367. 
MALLARD,  51,  65. 
Manganin,  222. 

in  galvanometers,  131. 
MARIOTTE,  law  of,  13,  27. 
Marquardt  porcelain,  181. 
MARSH,  170. 
MARVIN,  234,  277. 
MASCART,  403. 
MATHIAS,  364. 
MEIER,  8 1,  83,  84,  85. 
MEISSNER,  376. 
Meldometer,  n,  273,  361. 
MELLONI,  267. 
.Melting  points,  6,  189. 

certified,  458. 

methods  for,  179,  183,  188,  342,  343. 

of  chemical  elements,  Table  II,  Ap- 
pendix. 

of  salts,  190,  366. 

pyrometry  based  on,  365. 

standard,  433. 


Mercury  thermometer,  scale  of,  16. 

standards,  44. 
MESURE,  348. 
Metals,  certified,  458. 
MENDENHALL,  268,  328,  334,  342,  444, 

445- 

MEUTHEN,  92. 
MEYER,  328,  446. 
MICHELSON,  276,  277. 

MlLLOCHAU,  455. 
MOISSAN,  60. 
MOREWOOD,  366. 

MORSE,  vii,  10,  324,  325,  329,  330,  336, 

346,  426. 
Moss,  59,  434. 
MOULIN,  247. 
MYLIUS,  60. 

Naphthaline,  boiling  point,  450. 
National  Physical  Laboratory,  457. 
NERNST,  253,  342,  459. 

melting  points,  8,  442,  443,  444. 

optical  pyrometry,  294,  319,  337. 
NEVILLE,  440,  452. 

melting  points,  6,  199,  438,  439,  441, 

442,  445- 
resistance  pyrometer,  194,  199,  202, 

231. 

NEWTON,  245,  265,  266. 
NICHOLS,  337,  338. 

Nickel,  in  resistance  pyrometers,  234. 
in  thermocouples,  103,  106,  167,  ?68, 

170,  174. 

melting  point,  445,  446. 
specific  heat,  93. 

Nitrogen,  as  thermometric  gas,  18,  20, 
21,  22,  23,  25,  31,  33,  35,  68,  80. 

NOBILI,  267. 

NORDMANN,  346,  353,  455- 
Normal  scale  of  temperatures,  21. 

thermometer,  3,  22 
NORTHRUP,  ratiometer,  223. 
NOUEL,  348. 
NUTTING,  255. 

OBERHOFFER,  99. 
specific  heats,  92. 


INDEX 


505 


OHM,  118. 

Ohmmeter,  218. 

Optical  pyrometer,  10,  291,  296. 

applications,  338. 

calibration,  302,  306,  311,  316,  327, 

33i. 

corrections,  257,  336. 

errors  of,  306,  319,  322. 

extension  of  scale,  336. 

measurements  with,  305,  326,  338. 

of  D.  Berthelot,  85. 

of  Crova,  350. 

of  F6ry,  311,  354. 

of  Henning,  330. 

of  Holborn  and  Kurlbaum,  324,  337. 

of  Le  Chatelier,  296. 

of  Mesur6  and  Nouel,  348. 

of  Morse,  324,  329. 

of  Shore,  311. 

of  Wannerj  314,  324. 

range,  32^: 

scale  of,  306,  307,  336,  434. 

stellar,  353. 

use  of,  344. 
ORTON,  375,  376. 
OSMOND,  191,  384. 
OTTERHAUS,  155. 

Palladium,  latent  heat,  65. 

in  resistance  pyrometer,  233. 

in  thermocouples,  101,  103. 

melting  point,  64,  67,  185,  201,  272, 
442. 

specific  heat,  64. 
PALMER,  170. 
PARVILLE,  150. 
PASCHEN,  vii,  268. 

radiation  laws,  246,  248. 
PAUL,  129,  131,  158,  159,  218. 
PEAKE,  176,  426. 
PECHEUX,    thermoelectric    pyrometer, 

167,  168. 

PELLIN,  128,  298,  406,  408,  420. 
PELTIER,  109,  in,  131,  269,  270. 
PERROT,  61,  63,  68. 

expansion  of  gases,  25. 

gas  thermometer,  70. 


PERROT,  melting  points,  439,  440. 
PERRY,  122. 

PETAVEL,  200,  443,  454,  462. 
PETIT,  246,  265,  266,  267. 
Phosphorus,   effect  on   thermocouples, 

107. 

Photometer,  296,  352. 
PIONCHON,  specific  heats,  92,  93. 

PlRANI,  259,  328,  341,  446. 

PLANCK,  vii,  340,  353,  355,  455. 

radiation  laws,  251. 
Platinum,  alloys  of,  54,  in,  171. 

expansion  of,  54. 

latent  heat,  64. 

melting  point,  64,  67,  115,  294,  334, 
442. 

resistance  pyrometer,  194,  203. 

specific  heat,  63,  91. 

thermocouples,    67,    101,    105,    in, 
171. 

thermometer  bulbs,  54,  61. 
Platinum-palladium,  thermocouple,  101, 

i73- 

Platinum-iridium,  expansion  of,  43,  55. 
gas  thermometer  bulbs,  39,  43,  69, 

75,  76. 

thermocouple,  103,  105,  173. 
Platinum-rhodium,    gas     thermometer 

bulbs,  54,  76. 
thermocouple,  67,  105,  108,  114,  116, 

173- 

PLATO,  91,  367. 
Porcelain,  expansion  of,  58,  59,  67. 

gas  thermometer  bulbs,  44,  57, 62, 63, 
65,  66,  68. 

insulators,  151. 
Potentiometer,  138. 

for  radiation  pyrometers,  280. 

for  resistance  pyrometers,  214. 

for  thermocouples,  139,  143. 

precision  requirements,  141. 
POUILLET,  iii,  9,   14,  16,  54,  91,   137, 
263,  264,  265,  441. 

gas  pyrometer  of,  61. 

pyrheliometer,  262. 

scale  of,  3,  62,  346. 

thermoelectric  pyrometer,  101,  121. 


INDEX 


PRIM,  342. 

PRINGSHEIM,  vii,  338,  340,  454. 

radiation  laws,   238,   246,  248,   250, 

251,  252,  352. 
PRINSEP,  365. 
Purimachos,  151. 
Pyrheliometer,  262,  276. 
Pyrometers,  9. 

calorimetric,  10,  89. 

contraction,  i,  n,  357. 

dilution,  377. 

expansion,  360. 

fixed  focus,  285. 

fusing  point,  365. 

gas,  9,  37- 

optical,  10,  292. 

pneumatic,  378. 

radiation,  10,  262. 

recording,  12,  286,  381. 

resistance,  10,  194. 

spectral,  330. 

standardization  of,  432,  457., 

stellar,  352. 

sentinel,  366. 

thermoelectric,  n,  101. 

transpiration,  378. 

vapor  pressure,  380. 

various,  357. 

viscosity,  378. 

Quartz,  expansion  of,  60. 

gas  thermometer  bulbs,  60,  70. 

insulators,  151. 

mercury  thermometer  bulbs,  365. 
QUEEN,  408. 

Radiation,  laws  of,  4,  238,  272. 
application  to  pyrometry,  253,  261, 

266,  291. 

constants  of,  247,  250,  251,  253. 
of  Kirchhoff,  243. 
of  Le  Chatelier,  303. 
of  Planck,  251. 
of  Rasch,  294. 
of  Stefan,  245. 
of  Wien,  249,  251,  254. 


Radiation,  intensity  of,  238. 

monochromatic,  291,  295. 
Radiation  pyrometer,  10,  257,  261,  277, 
284. 

calibration,  288. 

computation,  290. 

errors  of,  283,  287. 

focusing,  282,  288. 

of  Brown,  284. 

of  Fery,  277,  281,  284. 

of  Foster,  284. 

of  Thwing,  284. 

results  with,  285. 

use  of,  285. 
Radiobalance,  269. 
Radiomicrometer,  267,  271. 
Radiometer,  267,  275. 
RAMSAY,  367. 
RANDALL,  54. 
Range  control,  426. 
RANKINE,  18. 
RASCH,  240,  295,  304,  444. 
Ratiometer,  221. 
REAUMUR,  scale  of,  3. 
Recording  pyrometers,  12,  381. 

accessories,  426. 

autographic,  408,  417. 

curve  tracer,  413. 

drum  recorder,  410. 

gas,  385- 

methods  of,  384,  390,  395,  396,  403. 

multiple,  428. 

of  Benedicks,  421. 

of  Callendar,  386,  390. 

of  Carpentier,  394. 

of  Charpy,  403. 

of  Dejean,  399. 

of  Le  Chatelier,  396,  420. 

of  Leeds  and  Northrup,  393. 

of  Kurnakow,  403. 

of  Roberts- Austen,  401,  415. 

of  Saladin,  418. 

of  Schmidt,  404. 

of  Siemens  and  Halske,  392,  408. 

of  Wologdine,  407. 

photographic,  396. 

radiation,  423. 


INDEX 


507 


Recording  pyrometers,  resistance,  386. 

semiautomatic,  413. 

thread  recorder,  410. 

thermoelectric,  395. 

REGNAULT,  iii,  iv,  v,  10,  14,  17,  18,  19, 
20,  29,  51,  62,  85,  97. 

boiling  points,  6,  434. 

experiments  of,  16,  84. 

specific  heats,  92. 

thermoelectric  pyrometer,  101. 
Reichsanstalt,  Physikalisch  Tedmische, 
vii,  7,  54,  68,  75,  127,  139,  342,  372, 
376,  452,  457. 
Resistance  pyrometer,  10,  194. 

as  standard  pyrometer,  227. 

calibration,  224,  231,  234.  t 

compensation  for  leads,  208,  230. 

constancy  of,  232. 
-construction  of,  202,  218. 

direct  reading,  218. 

errors  of,  228. 

formulae  for,  197,  201,  225. 

heating  by  current,  228. 

insulation,  221. 

industrial  installations,  234. 

industrial  forms,  204,  216,  218. 

lag  of,  229. 

-laboratory  forms,  203,  208,  218. 

methods  of  measurement,  207,  218, 
230,  386. 

nomenclature,  201. 

of  impure  platinum,  231. 

of  palladium,  233. 

precautions  with,  206,  228,  234. 

protection  of,  235. 

recording,  386. 

reduction  tables,  220,  232. 

results  with,  226. 

scale  of,  197,  226,  235. 

size  of  wire,  206,  231. 

sensitiveness,  216. 

use  of,  234. 
Rhodium,    in    resistance    pyrometers, 

234- 

in  thermocouples,  105,  in,  116. 
melting  point,  444. 
RICHARD,  403,  404,  408. 


RICHARDS,  99. 

specific  heats,  94. 
ROBERTS-AUSTEN,  12,  151,  191,  407. 

recording  pyrometers,  382,  401,  403, 
404,  405,  415,  416. 

ROSE-lNNES,  21,  35. 
ROSENHAIN,  384. 
ROSETTI,  10,  245,  267,  273. 

radiation,  265,  266,  267. 
ROTHE,  418,  434. 
Roux,  402. 
RUBENS,  251,  269. 
RUDOLPHI,  171. 

RUER,  446. 

RUFF,  367,  443. 

RUTHENIUM,  in  thermocouples,  174. 

SAINTE-CLAIRE-DEVILLE,  iii,  9, 16,  51, 

54,  57,  62,  63,  83,  85,  438. 
SALADIN,  420. 

recording  pyrometer,  407,  418. 
Salts,  melting  points  of,  190,  365,  450. 
Scales,  color,  346. 

control  box,  427. 

electrical  resistance,  197,  228. 

gas,  3,  21,  23,  432. 

gas  scale  corrections,  30. 

mercury-in-glass,  17,  364. 

normal,  21,  24,  37. 

of  the  Reichsanstalt,  7,  8. 

practical,  13,  17. 

standard,  13,  21,  37,  432. 

thermodynamic,  3,  4,  13,  26,  31,  35, 

432- 
thermometric,  2,  432. 

SCHAFFER,  380. 
SCHEEL,  6l. 
SCHEIMER,  355,  455. 

SCHMIDT,  404,  405. 

SCHOTT  and  GENOSSEN,  298,  335,  337. 

SCHUKAREW,  99. 
SCHULZE,  340. 

SECCHI,  265. 

SEEKECK,  101. 

SEGER,  n,  369,  370,  375,  376. 

fusible  cones,  368,  372. 
Seger  cones,  n,  366. 


INDEX 


Selenium,  in  pyrometry,  355. 

SEXTON,  437. 

SHENSTONE,  60. 

SHORE,  312. 

SIEBERT  and  KUHN,  60,  365. 

SIEMENS,  iv,  10,  210. 

calorimeter,  96. 

resistance  pyrometer,  194,  195,  209. 

scale  of,  3. 

SIEMENS  and  HALSKE,  12, 127, 131, 133, 
145,  208,  210,  392,  408,  409,  417, 
420,  425. 

Silicon,  effect  on  thermocouples,  107. 
Silver,  in  thermocouples,  172,  174. 

melting  or  freezing  point,  62,  65,  67, 

88,  440,  445,  448. 
SMITH  (F.  W.),  230,  231. 

SOMERVILLE,  167. 

SOSMAN,  viii,  68,  77,  79,  81,  in,  114, 

155,  160,  253,  440,  456. 
expansion  of  metals,  55.. 
gas  thermometer,  75. 
melting  points,  8,  438,  439,  441,  442, 

445,  446,  451. 

thermoelectric  pyrometer,  115,  116. 
Specific  heat  of  metals,  91. 
pyrometer  (see  Calorimetric  pyrom- 
eter). 

Standardization  of  pyrometers, 43 2, 45 7. 
laboratories  for,  457. 
optical,  302,  331,  336. 
radiation,  288. 
resistance,  f!6,  227,  236. 
thermoelectric,  178. 

Standards,  Bureau  of,  7,  212,  252,  325, 
342,  364,  372,  375,  447,  457,  45», 
459- 

Standard  temperatures,  5,  9,  13,  433. 
Standard  cells,  136. 
STANSFIELD,  in.  . 

melting  points,  438,  442,  445. 
thermoelectric  pyrometer,  113. 
STEFAN,  247,  250,  253,  254,  272,  273, 

278,  280,  289,  455- 
radiation  laws,  245,  246. 
STEIN WEHR,  211. 
STEWART,  338. 


String  galvanometer,  421. 
Sulphur,  boiling  point,  75,  433. 
Sulphurous  acid,  as  thermometric  gas, 

18. 
Sun,  temperature  of,  261,  273,  455. 

Tables,  489. 

TAIT,  in. 

TAMMANN,  367. 

Tantalum,  lamp  temperature,  341. 

melting  point,  446. 
Telescope,  pyrometric,  277. 
Temperature,  alarm,  431. 

black-body,  242. 

definition  of,  2. 

equivalent,  243. 

of  flames,  338. 

of  glow  lamps,  340. 

of  industrial  furnaces,  191,  346,  388. 

of  stars,  352,  355. 

records,  381. 

within  furnaces,  342. 
Temperature  coefficient,  of  galvanom- 
eters, 131. 

of  metals,  171,  173,  194. 

of  gases,  17,  25,35. 
Thalpotassimeter,  380. 
Thermocouples  (see  also  Thermoelectric 
pyrometer). 

annealing,  104,  149,  173. 

Becquerel  effect,  109. 

calibration  of,  178. 

chemical  changes,  107. 

cold  junction,  154,  155. 

compensating  leads,  156,  176. 

compound,  275. 

constancy  of,  159,  164. 

effect  of  reducing  atmosphere,  107. 

inhomogeneity  of,  106,  161. 

insulation  and  protection,  149. 

junction  of  wires,  148. 

neutral  point,  in. 

of  base  metals,  165,  193. 

of  complex  alloys,  170. 

of  Noble  metals,  171. 

reproducibility  of,  164. 

resistance  of,  118,  171. 


INDEX 


509 


Thermocouples,  thermoelectric    power, 
161,  166. 

tensile  strength,  173. 

Thomson  effect,  109. 

types  of  mounting,  151,  i$& 

parasite  currents,  106. 

Peltier  effect,  109. 

Thermodynamic  scale,  3,  4,  13,  26,  31, 
35,  432. 

ice  point  on,  34. 

Thermoelectric     pyrometer    (see    also 
Thermocouple),  n,  101. 

advantages  of,  102. 

applications  of,  191,  192. 

base  metals,  116,  165. 

choice  of  couple,  105. 

experiments  with,  102. 

extrapolation,  115,  174. 

formulae,  102,  108,  HI. 

galvanometers  for,  118,  120,  131. 

methods  of  measurement,  116,  118, 

i35,  i79- 

potentiometers,  135. 

recorders,  393. 

Thermoelectric  telescope,  277,  282. 
Thermogage,  324,  329. 
Thermometer,  mechanical,  360. 

mercury  for  high  temperatures,  362. 

standard  gas,  38. 

standard  mercury,  44. 
Thermophones,  376. 
Thermopile,  266,  268,  269. 
Thermostats,  430. 
THOMPSON,  342. 
THREW,  379. 
THURMEL,  352. 

THWING,  132,  157,  170,  284,  286,  408, 
411,418. 

TlLDEN,  91. 

Tin,  freezing  point,  445. 
TOEPFER,  403. 
TORY,  200,  202. 
Total  heat  of  metals,  91. 
TRAVERS,  60,  226. 
TROOST,  51,  62,  83,  85. 
TUCKER,  342. 
Tungsten  furnace,  462. 


Tungsten    furnace,  lamp   temperature, 

341- 

melting  point,  446. 
TUTTON,  59. 
TYNDALL,  246. 

UHLING-STEINBART,  12,  378,  379. 

Vacuum  furnace,  461. 
VALENTINER,  56,  68,  247,  253. 

gas  thermometer,  75. 

melting  points,  7,  440,  442,  443,  444. 
VAN  DER  WAAL,  34. 
VAN  DER  WEYDE,  444. 
Vapors,  metallic,  107. 
VERY,  454. 
VILLARD,  60. 

VlOLLE,  iv,  10,  263,  265,  454. 

actinometer,  263. 

gas  scales,  23. 

gas  thermometer,  63. 

melting  and  boiling  points,  5,  64,  65, 
438,  439,  44i,  443,  444- 

specific  heats,  63,  91. 
Viscosity  (pyrometers),  376. 
Visibility  curve,  256. 
VOGT,  368. 


WAIDNER,  viii,  10,  79,  212,  253,  330, 

334- 
freezing  and  boiling  points,  7,  199, 

226,  435,  437,  438,  44i,  442,  443, 

444,  445,  446,  452,  454,  459. 
melting  points,  9. 
optical  and  radiation  pyrometry,  240, 

243,  253,  307,  320,  323,  332,  335, 

337,  346. 
resistance  pyrometer,  199,  200,  205, 

228,  231,  232,  233. 
thermoelectric  pyrometer,  185. 
WANNER,  vii,  10,  320,  323,  336,  342, 

346,  454- 
optical  pyrometer,  312,  315,  319,  322, 

324. 
WARBURG,  253. 


INDEX 


WARTENBERG,  253,  255,  259,  319,  462. 

melting  points,  8,  442,  443,  444,  445, 
446. 

optical  pyrometry,  337. 
WASSMER,  68. 

boiling  points,  7,  437. 
Wedges,  optical,  313,  333. 
WEDGWOOD,  iii,  i,  n,  358,  359. 

pyrometer  of,  i,  357. 

scale  of,  3. 

WEISS,  specific  heats,  92,  93. 
WENNER,  146. 

WESTON,  129,  131,  136,  137,  140,  212. 
Wheatstone  bridge,  for  resistance  py- 
rometer, 208,  211. 
WHIPPLE,  224,  286. 
WHITE,  99,  146,  161,  451. 

specific  heats,  91,  94. 
WIBORGH,  83,  376. 

WIEN,  vii,  57,  200,  250,  253,  254,  255, 
294,  303,  306,  315,  317,  328,  334, 
337,  339,  340,  342,  352,  354- 


WIEN,  gas  pyrometer,  67. 

melting  points,  7,  67,  439,  441,  443, 

455,  456. 

radiation  laws,  239,  249,  251. 
resistance  pyrometer,  196,  199,  233,. 

234- 
thermoelectric  pyrometer,  112,  119,. 

126. 

WILSING,  355,  455. 
WILSON,  271,  272,  273,  306,  454. 
WOLFF,  137,  145,  174. 
WOLOGDINE,  407. 
WRIGHT,  185. 


YOUNG,  273. 


Zinc,  boiling  point,  62,  63,  65,  66,  88, 

438. 

melting  or  freezing  point,  62,  437, 
445- 


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